All of my games are appearing out of order, even when I switch to alphabetical order, Ultima will appear before fallout and Might and magic. and no matter what order I choose (oldest/newest first, alphabetical etc) they appear out of order in the library. This happens in both GOG Galaxy and on the actual website online.

I believe this is the thread you're looking for:

https://www.gog.com/forum/general/my_account_seems_to_be_playing_musical_chairs

Check the thread drmike linked - it gives more details and the fact you are not alone.

The TLDR version:
It has been happening to all of us for a few days now.
GoG is aware of it and supposedly working on it.
Your games are all still there, you just may have to use the search function for now - which still appears to work.