Saves wiped??

@theowlgang  Please look within OneDrive's Documents folder for any Sims 4 data.  If you find your saves or anything else, you can move them back the local version of Documents > Electronic Arts > The Sims 4, and the game should read them again.

You can also search your computer for any stray saves.  Open a File Explorer window, click This PC in the left panel, and enter Slot_ in the search field in the upper-right corner.  If you find a save, select it and click the "Open file location" menu button, and you'll jump to wherever the save is, which will hopefully be where the rest of your content is as well.  Here again, you can move it back to the proper location, and the game should read it.

As for why this happens, if it's a OneDrive issue, there are ways to change the settings to try to prevent it.  But the best prevention of all is frequent backups so you never get caught with just one copy of your Sims data.  You can copy the entire Sims 4 folder to an external drive or USB stick, or if it's small enough, upload it to a free filesharing service like Google Drive or Dropbox.  In either case, it's a good idea to zip the folder first (right-click and select Send to > Compressed file), then unzip it if and when you need to use it.  You can delete older backups as you go along.