Help! All my Sims disappeared!

If I'm understanding your first post, you still have your saved games in your Sims 4\saves folder.
C:\Users\Username\Documents\Electronic Arts\The Sims 4\saves

As that's what is says in the first post. It may be the game isn't reading the save files correctly. - I've had this happen to me a few times and what has helped is renaming the save files in the save folder.

Have you tried renaming your saves? If you have an old auto save file (Slot_00000001.save) it's safe to delete as the game doesn't use it anymore. To rename your save, remove the version number. - .ver0 or .ver1 by deleting the .ver1 on the end.

For example:

Slot_00000002.save.ver1

Rename it to:
Slot_00000002.save

You may want to give it a different number if you already have a save 2 in your folder. The game should then recognize the save file again.

If the above doesn't help, Crinrict has some very useful tips on saved games disappearing:

Save Games disappearing
https://sims4.crinrict.com/en/2014/09/save-games-disappearing/

---

@jpkarlsen wrote:

So the Sims 4 /Mods and Sims 4/saves on OneDrive and on your local HD are now identical but you still have the problem?

---

Yeah, I do. I followed all the steps, and I know everything's the same, but still nothing has changed.

Then the game must use a third location to store its Sims 4 folder with subfolders.

Please move the Sims 4 folder both from the Local Documents and from OneDrive to somewhere else. I suggest you make two folders on your desktop named fromonedrive and fromlocal and place the local and OneDrive Sims4 folder in respective folders so you dont confuse them.

Then start the game. If the Sims 4 folder is not recreated in either of those places we know for certain that neither of the places are used by the game and all that is left is to search for where it is stored.

If that's the case, please post the screenshot path (picture above) and it will probably make it easier for you with to find where the game is saving things.