Saved games deleted after restarting

@cringeyjesus  You can try searching your hard drive for any stray saves.  Open a File Explorer window, click This PC in the left panel, and enter Slot_* (with the asterisk) in the serch field.  If you do find saves, you can put them back in Documents\Electronic Arts\The Sims 4\saves, and they should show up in-game.  The ones with a .verx on the end are backups, and you'll need to delete the .verx to get them to load, but you don't need to do that unless the non-backup saves don't load.

You can also try looking in OneDrive; it's possible your Sims 4 folder was uploaded at some point.

@cringeyjesus  First, create a folder (name it Backup Saves or something) on your desktop, and move the saves you found to that folder.  Then copy them (right-click and select Copy), open Documents\Electronic Arts\The Sims 4\saves, right-click in the empty space, and select Paste.  That way, you'll have backup copies of your saves in case you need them later.

If you get a message about overwriting files, it's because one of the saves you're moving has the same name as a file already in the folder.  You can rename one of the affected saves so it has a unique number instead.  Just make sure it's still labeled the same way, e.g. Slot_00000005.save or Slot_00000012.save.