My Sims 4 library disappeared and I don't know how to get it back

@alltimejess1849  Right, so your Sims 4 content was moved only to Microsoft OneDrive's cloud storage, with no copy left on your hard drive.  When you signed back into your Microsoft account, it synced your data again.

I agree with the other people who say your OneDrive account shouldn't be allowed to affect your Sims games, and there's a simple way to make sure you don't lose the data again.  Before you do anything else, make a copy of the entire Sims 4 folder in Documents\Electronic Arts and put it in your Downloads folder, which OneDrive doesn't touch.  Then sign out of your Microsoft account again, or else just tell OneDrive not to sync the contents of Documents:

https://support.microsoft.com/en-us/office/choose-which-onedrive-folders-to-sync-to-your-computer-98b8b011-8b94-419b-aa95-a14ff2415e85

You may have to pause syncing and/or cancel the current sync first:

https://support.microsoft.com/en-au/office/how-to-cancel-or-stop-sync-in-onedrive-4885c27e-3d89-4d69-be75-2646c71367d3

Once that's done, the original Sims 4 folder will probably have been removed, but that's fine, you can just move the copy you made back to Documents\Electronic Arts and pick up where you left off.