I purchased No Man's Sky on November 26th, 2016 and I've installed it, played it and patched it without trouble since then. Today I went to update via Galaxy and now the game is no longer in my library and I cannot reinstall or update it, is this a known problem? Any help would be appreciated.

I'd say the most likely cause is the new update which will probably arrive soon. Maybe they're currently messing around with their database.

If your game still won't work/show up properly within a few days, best contact support about it.

there's no reason a game update would mess with GoG database.
I suggest he contact an administrator immediately.

Especially if he hasn't lend the game to a "friend".

But before that IU really suggest he goes througth his entire library to double check for no man's sky presence and
search his e-mail for NMS buying ticket.

I can try that, thanks, but even logged into the website its not showing in my game shelf. Not cool. And no, as the other poster mentioned, I haven't "lent" it out. It is installed on my desktop at home and my laptop, that's all.

I understand, in this case, contact support and hopefully they will be able to sort it out reletivly quickly.

You can still use the conventional GOG Downloader. Download it, enter your profile.
Go in your store page, click on NMS, chose More > Download Links. Chose the main installer
This will fire up your GOG downloader and start dl.
After that, also chose the patches (if any)

Hope this helps,

Its no longer in my library and therefore not available to download, through the site, Galaxy or the downloader. Didn't know if this was a known problem with the new patch

Don't miserderstanded me,
I , one time, lend my game to friend who puted internet.
So I had to explain why a copy signed with my signature
was on free download on torrent websites.
It had a taste of betrayal
It wasn't funny at I don't wish it to happen to anyone.

Lending your Gog.com games to friends isn't against GSC
It's not warning this friend to not putting the copy on internet neither giving to
another friend ( the friend of a friend ) .

The problem cannot in anyway be related to the titan update
contact a community manager ASAP to resolve the problem.

Oooooo that sounds terrible, definitely no laughing matter, I am sorry. No I haven't done anything like that. I like that GOG is DRM free so that I can install some of my titles on my desktop and the kids can play them. They never get my password and that's not against any TOS-----quite the opposite, I thought that was something GOG encouraged; the ability to install on more than one of your devices. I've been waiting to hear back but nothing so far, definitely reached out the them right away. I guess I'm glad that its not a widespread issue? Just hope we can get to the bottom of it

Nothing to do with No mans sky, but similar.
I bought Rebel Galaxcy and I t was in my library, then when I finally felt like playing it weeks later it was gone.
GOG asked for any proof I paid for it, you can find that in your account details.
After a few days it was back, so this problem of yours does happen.
Good luck =)