Do you have any Mods or CC? If so move your Mods folder to the Desktop and delete localthumbcache.package. Then test the game.
can you tell me what deleting that would do? I don't want to delete anything without knowing what it does
It clears the cache. The game will create a new localthumbcache.package file.
I really know nothing about computers, sorry that I sound dumb, but I don't know what a localthumbcache.package is or what it does. What does it store? And will deleting it bring back my game?
localthumbcache.package is a cache file that can get big fast. It gets recreated after it's been deleted. It's a good idea to delete this file when having issues with the game and/or adding/removing custom content or mods.
It may not bring back your game, but it helps with troubleshooting the issue.
If you want to learn more about Sims 4 user files, see the link below:
Sims 4 User files explained
https://sims4.crinrict.com/en/2014/09/user-files-explained/
