YoungerSatan Does "save as" work?
YoungerSatan and cchant86 Is all of your progress gone, or only some of it? For example, the sim-day and time might be stuck, but some build progress remains, or your sims have new items in their inventories; or it could be that the entire save reverts to its previous state.
Please also let me know what antivirus you use, and whether OneDrive is running on your computers, even if you never use it.
cchant86 I've merged your other post into this thread to keep the relevant info together. Since you mentioned there the actual size of the save, which I agree is alarmingly large, please open that save and see how many sims are in Other Households and whether any of those households look corrupt.
If you're willing to temporarily add a mod, it would be useful to add Show Sim Info by frankk (NOT the MTS version, which is outdated). Click a sim and select Search All Sims, and you should get a window that lists all sims in the game, including hidden ones. The total might be excessively high.