I'm having a terrible problem and would appreciate any advice that anyone might have! I've been using Linux Mint 20 with Cinnamon for a long time now, and I have it installed on a few partitions of a drive which also has Windows 10 Professional. I've installed it on various occasions and it never interfered with Windows in any way. Now I just reinstalled it on one partition, and I updated GRUB and everything the usual way, so that's all fine, and all Linux partitions boot and work fine (except that the new one can't access the Internet for some reason, but I'm not sure why).

But now when I boot Windows it doesn't work! I didn't alter that partition at all (it's not even adjacent to the partition that I changed), and GRUB is still doing its job fine, but when Windows loads it just gets to where it displays the desktop as plain wallpaper with no icons, and the taskbar with very little on it, and wherever I click, if it has any effect at all it just fades the screen lighter and turns the cursor into a spinning blue circle. Then if I click again sometimes it goes back to normal but just alternates between those two states. I also can't access the Start menu. I can do Ctrl-Alt-Del but I can just sign out, and if I try to display the Task Manager nothing happens. If I sign out it always says that it needs to end tasks before I do, or I can force it, but it seems to be different tasks every time and I don't recognize them. I'm CERTAIN there's no virus or anything like that, and this started happening right after I reformatted the other partition and reinstalled Linux Mint.

I looked up some information and found that it might be some problem with the user profile settings having gotten messed up somehow, or something like that, so that the profile still exists and can log in but has no access to any files or anything. It suggested to go into safe mode and change some things, but when I got into safe mode, I had the same problem, except that the wallpaper looks black, and when the cursor starts spinning I get a message that wants me to end a task, though either it doesn't specify what the task is, or it's referring to Windows itself - I'm not sure which. If I do end the task nothing happens and it goes back to how it was, and I can tell it numerous times in a row to end the task and still nothing happens.

I could try a startup recovery option but I'm not sure whether that would do any good or not, though I hope it wouldn't make anything worse by simply changing some fundamental options in a way that might somehow not be appropriate for the circumstances.

I should point out that I already had an annoying problem that when I pressed the Window key (between Ctrl and Alt) it would show an error that expected me to sign out (though I could leave the error on the screen and go about my business until I was ready to restart), and I don't remember whether clicking the Start menu worked but it might have caused the same error. I think it was caused by some weird shut down or something going wrong once, but I don't remember what, and it never caused any other problems than that. I wasn't sure how to fix it, and I never really bothered, because I only use Windows for playing games, so I just install them, use the shortcuts that I need and pretty much ignore everything else.

But now I'm wondering whether that's the reason why the start menu doesn't work while I'm encountering this problem, or whether it wouldn't work anyway (since nothing else does, either).

So anyway, does anyone have any idea what to do? I'd really like to avoid having to reinstall Windows, because if I do that then I'll probably have to take it into the shop to have it done, since I'm not sure whether I have the proper validation code (these days it seems like they don't want people to be able to authenticate their own copies of Windows, and they only trust the OEMs and other such professionals to do it - stupid, unconscionable Microsoft!).

I'm not sure. Probably can rule out bootloader stuff if Windows is booting. Are you able to create a new Windows user and log into that account?

You might also try booting into the windows repair thingy which I think you can access w/ the setup media.

In the future if possible it's probably best to have them on separate physical drives.

Well I could try the repair tool, but like I said, I'm not sure whether it would make it better in this circumstance or possibly worse, only because I'm really not sure what's wrong with it in the first place.

I'm not sure how I could make a user since I can't actually do anything at all once I'm in Windows. Is that something I can do before I sign in?

It might be good to put it on two different drives, but I only have one. Also, if I did that, how would GRUB be able to boot both of them, or would I have to go to the UEFI boot menu to do it?

I don't know exactly how it works. In my case both linux and windows are sharing an EFI partition but the operating systems are on separate drives. On startup I use the EFI boot menu to choose which to start. 99% of the time I want Linux so that's what I have set as the default in the BIOS.

If you can get to a cmd prompt you can try:
net user <username> <password> /ADD

Try ReFind. If you might. Grub is Scary, and I try to avoid it whenever I can.

Hello HeresMyAccount, hope you're doing fine overall!
Im so sorry that you're going through this trouble. It's really worrysome and annoying.

I can't help much because I also have no idea what that might be, albeit I also suspect it's a file permissions issue, because something like this happened to me before and I found a way around to change all files permissions to a new user I've created.
Unfortunately, that happened a while ago and I can't remember what I did back then to do this.
But I remember I searched online and read forums to find how to do that.

If I find something that might help you I will post it here later.

In any way, if you're able to solve the file issue or take control over the system but not the files and need your Windows 10 product key, I'd like to point out that you can use CMD and Powershell to find your product key if your computer came with a Windows 10 installed.

Here are the commands:

For Command Prompt (CMD), as Administrator: “wmic path softwarelicensingservice get OA3xOriginalProductKey”
For Powershell, as Administrator: "(Get-WmiObject -query ‘select * from SoftwareLicensingService’).OA3xOriginalProductKey"

--edit:

Another solution or troubleshooting would be burn a Windows 10 ISO into a USB drive and boot into it using your PC boot options.

Normally you can access your system boot load order to change it trough GRUB.
It's the option that says something about start using your system UEFI settings.
On your PC UEFI settings, you can change it from the one you're using as first, to your USB with Windows 10 ISO burned (It must be connected beforehand.).
Boot through it and it might give you options for troubleshoot and recovery If I remember correctly.

(Do not take my word for granted here, I might be wrong in this, please research for other experiences on what I will write down: )
I believe there's no risk in breaking your Linux Mint installation on the same Drive because Windows 10 recognizes Mint as Ubuntu and won't change a thing in it, and you can select what partition on your drive you want to repair / recover (do not select ubuntu one or it will, well, probably install Win10 on it), albeit it will change your Boot load order and instead of booting on Linux Mint's GRUB, It will start booting directly into Windows 10, but again, you can change boot order on your pc UEFI settings and set it to 'ubuntu' / Linux Mint's GRUB again.

Hm.... Best of luck with this mate.

I'll reply, but first I should say that I know the files on the Windows partition aren't corrupt, at least not generally, if at all, because I can still access them from within Linux.

EverNightX, that's weird that it would boot two different drives from one, but then couldn't that ultimately cause the same problem? I don't know how to get a command prompt, because I can't seem to get any program to run, or even see that they exist in the first place.

Darvond, why is GRUB scary? I've always used that, and it's what's included with Mint, so I'm not really sure I can change that, or if it's a good idea to try, but I would like to know why you don't like it, because I'd want to know if there's somwthing wrong with it.

Keys, thanks for the commands, but I can't seem to get into CMD or Powershell, or at least I don't know how, if it's even possible given the circumstances. I already have my USB drives bootable (that's how I install Linux). Also, I hadn't been worried that Windows would mess up Linux so much as that Windows might mess up itself in such a way that would make it even harder to repair than it already is, but is there any danger of that happening?

Not really. One central place to see all the boot options, and then one is selected to load. It's under advanced recovery options:
https://www.youtube.com/watch?v=LZqrjjJWJdo

Fortunately since you have access to the windows drive from Linux you can backup what you need and reinstall windows again if it's salvageable.

Oh, it is? Alright, thanks. But still, I'd like to avoid having to reinstall Windows if possible, so I'll keep that as an option if I get desperate enough to do it, but it's nice to know that at least it is an option.

Right. But creating a new user isn't going to hurt anything.

It should work. Create the new user and try logging in as it. Really, what do you have to lose?

Yeah, I'll try that next time. But I need to make sure that I'll have administrator privileges, which is why I thought it might be better to adjust the settings of the existing user and just fix whatever's wrong, but I'm not sure how.

net user /add [username] [password]
net localgroup administrators [username] /add

You should be in an administrative cmd prompt when booting from install media.

Also on Windows it is better not to use an admin account for daily use but to borrow the powers when needed and login to the admin when you must, which is annoyingly often (like once a month for me when/if I use Windows regularly).

I tend to check memory and storage for faults when odd problems occur.
Hope you can fix it!