steam bugged

Do you have any filters on in your library?

Can you take a screenshot of your library drop-down? You're probably making it show you only the games you've installed? idk.

maybe u typed something in the search field by accident

Tried restarting Steam?

1. Check if you have selected a certain "category" of games that you might have closed (it's acalled a branch I think?)
2. Check if it's not filtered on "Recently played".
3. Set the filter to "Recently played", then back to "All games".
4. Restart steam.
5. opt in or out of a steam beta.
6. Contact steam support.