Getting A Fresh Start
1. Assuming you've made a backup in Desktop, go to your Tray and Saves folders. This is where your games and CAS should be.
*In case you haven't, to make a backup: copy and paste your The Sims 4 folder from Documents to Desktop*
2. Now that you know they're there, go into Documents and open the Electronic Arts folder, click on The Sims 4 and delete it.
3. Now start up your The Sims 4 game and then close it.
4. Go into Documents and there should be a new The Sims 4 folder in your Electronic Arts folder.
Resurrecting Your Old Stuff
*Make sure you try to run the game after you add anything to the new folder*
1. To get your stuff back into the game it must go into Documents, to do this simply copy and paste everything in the backup Tray and Saves folders into their respective folders in the new The Sims 4 folder in Documents.
2. Open The Sims 4 game, everything should be there. *If it doesn't open see bottom*
3. Have you got mods too? Check the backup Mods folder in Desktop to make sure it's all there.
4. Start slowly moving (not copying and pasting) your stuff from the backup Mods folder into the new Mods folder in Documents alphabetically in batches (for ease).
If Your Game Stops Opening
Uh oh, you've hit something broken!
*Make sure you try to open the game after you add anything to the new folder*
1. Take out the batch you just added to the new folder in Documents by placing in back into the backup folder (or deleting it if you already have it in the backup folder)
2. Now slowly put the batch back into the new folder but one by one.
3. Whenever it stops opening it means you've added something broken, so delete the thing you last moved to the new folder.
4. Continue this process as there might be more that is broken.
I really hope this helped! Happy Simming!