I had a problem similar to this a while ago. Unfortunately, the backup file was also corrupted, and I lost a family I had been working on for several months. Gutted. Since then, I rarely use the 'Save' option, and instead go to 'Save As', and I alternate between a few file names. So, when everyone has gone to bed on day 1, I save my file as (e.g.) Twinbrook 1. Then the next day when everyone has gone to bed, I save as 'Twinbrook 2'. Day 3 I swap back to 'Twinbrook 1' and so on. That way, the very worst that could happen is I lose 2 sim days of gameplay, rather than my entire game.