@morganmileham Unfortunately, at this point, the save is corrupted, and there's no way to recover it. Enough data on your sims has been lost that even if you could get back to your sims temporarily, the save would be unstable and likely crash and burn very soon.
The only thing you can do now is revert to a previous save, from before your sims moved. Even if you don't make backups yourself, the game keeps the files from the second to last time you saved. Inside your TS3 game folder (in Documents\Electronic Arts), there's a folder labeled Saves, and inside that, each save has its own separate folder, as do the backups. So if your save is called SunsetValley, the main save folder will be labeled SunsetValley.sims3, and the backup will be SunsetValley.sims3.backup. Change the name on the spare and delete the .backup extension (i.e. SunsetValley1.sims3), and this save will be available to load from the Main Menu.
In the future, the best way to move your sims from one town to another is to save them to the bin and start a completely new save. Be sure to quit to desktop before loading the new town; if you switch at the Main Menu, some data from the first save may transfer to the second and corrupt it. If you want to keep your sims' house, save it to the bin separately, after you've evicted your sims.
It's also a good idea to keep backups of your favorite saves. You can copy them elsewhere or just use "save as" to rename the save periodically, and the game will keep both the new save file and the old one under the previous name.
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I don't work for EA. I'm just trying to help fellow players with their games.
