kn013 EA support told you that this would be resolved in a few hours? That's... well, it's weird, and also a lot of other things.
I've merged your posts into the master thread for the Error 16 message, which is currently usually caused by OneDrive. Before going that route though, please create a new save and make sure Seasons is loading in-game. You don't need to save your progress, or even spend any time on it past going into live mode and confirming that the world does in fact have a current season (summer by default in a new save). If Seasons isn't loading, that's a separate issue and may explain why you couldn't reload the save.
Aside from that, the error you see on the Main Menu is somewhat generic—the save can't be loaded, maybe because an expansion is missing but maybe not. It could also be that your most recent save is damaged in some way. So please try loading the backup, if you can.
Open Documents > Electronic Arts > The Sims 3 > Saves and look for a folder called Sunset Valley.sims3.backup , as in, your save name with a .backup extension. Rename the save and delete the .backup (e.g. Sunset Valley 1.sims3), and it will be available at the Main Menu.
For the potential OneDrive-related part of this, it's best to either get rid of OneDrive entirely or at least exempt your Sims 3 data from syncing. Disabling or uninstalling OneDrive isn't usually sufficient; you'll need to move the Documents directory out of the OneDrive folder if it's currently there. Please see the section under the spoiler in this post for more information:
[Current Issue] Sims 3 "serious error" when loading save | EA Forums - 8322938