I've seen a lot of lost save files, both from friends and posted here.
The best way to make sure you keep a game save is to also periodically quit the game, go to your Documents folder > Electronic Arts > The Sims
Right-click the "saves" folder and select "copy". Then paste it somewhere else. (Perhaps make a "Game Backups" folder in your Documents)
This will put those save files out of reach of the game so they can't possibly get deleted unless you delete them yourself.
If a save goes missing, you can copy it back in from your backup folder.
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For save game that are broken now there is a way to get at least some of them back.
Yep, I hope everyones doing that as well! I only mention this as an additional step so the game can't possibly corrupt this backup.