Unless you have saved 5 times since this happened, you can recover an old save from before this happened. From the load game screen, there will be a button on your save that says "Recover Save" or "Recover Game" when you mouse over. When you click that, it will list the last 5 saves. When you choose one, it becomes a "save fork," your game with its most recent save will still be there, and then you will ALSO have Game (#) [Recovered].
And I've just noticed the original post date, I imagine it's too late now. Hopefully this will help someone else who had a sim die while out of their control though.