Mumumw Please see this guide that covers missing user data and how to search for it:
⚙️[PC] Sims 4 Missing User Content (saves, mods, etc.) | EA Forums - 12636780
As for why this happened now (recently), it's possible that your system drive is running out of space, and either a Sims 4 update or another download dropped the free space below the threshold for OneDrive to start pushing data to cloud storage. This could have been a temporary change, for example the EA App can budget 10-20 GB extra space for Sims 4 updates before relinquishing it once the update is done. That's only one possibility, but it's a relatively common occurrence.