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/
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