things are randomly invisible to me now

Example here:
http://cloud-4.steampowered.com/ugc/71250381382897794/9E60A9666153C8F7BD13FBA59DEC0B4E98E9F89E/

You can see the shadow of whatever is supposed to be there.

I have all the required games (At least I’m almost positive)

This is the map from the screenshot:

I’ve seen this issue before on ttt_fastfood_a6 with the shelves in the library, it seems to be an issue with the map and not your game. If it was a missing game it would have been an error. Try reloading the map once or twice and see if that fixes it.