I installed Fallout just fine on Windows 10, but when I try to play it I get a window that says:

"Could not load DirectX
This program requires Windows 95 with DirectX 3.0a or later or
Windows NT version 4.0 with Service Pack 3 or greater."

A few weeks ago I tried installing something that required me to fiddle around with DirectX 12 but it didn't work out and I ditched it. Now DirectX isn't working and I don't recall what I did to it, if I did anything at all.

I reinstalled DirectX and restarted my PC and the problem still continues.

HELP

Was it directx 9 that you reinstalled? Try reinstalling that, if it wasn't

You can get it from https://www.microsoft.com/en-au/download/details.aspx?id=34429

I tried reinstalling DirectX 10, not gonna lie, I don't know what I'm doing.

Tried your link, just ended up with this:

Ignore that message. Directx 9 will install on newer versions of Windows (and is necessary to run older games) It will also happily coexist with newer directx versions.

Edit: you might need to download a different update - try links further down the page from http://www.tomshardware.com/forum/id-2756506/directx-games-work-windows.html

I tried all those downloads, no luck.
I have to call it quits for today, but any future suggestions will be appreciated.

I've just realised that I linked you to a slightly older directx 9 in the first post - try https://www.microsoft.com/en-au/download/details.aspx?id=8109

It probably won't make much difference, but worth a try. I don't have Win10, unfortunately. You may have to wait for someone more familiar with Win10's quirks. I can only tell you that installing directx 9 should work, many Win10 users have done so successfully.