I have had that happen, I chalked it up to a corrupted save. It wouldn't load my household from the map screen though. The household it happened in, my sim was a scientist but I think he was already retired at that point.
If I'm not mistaken, if your save's slot number ends with a 'c' it means it's corrupted. At least most of my corrupted files had always ended with a 'c.' I'm not 100% sure if that's always the case though.
Can you save your sim to your Library and start a new save with them? I know career progress and skills carry over but I'm not sure if reward and bonus traits and degrees do. Could you make a note of all their achievements (and other things that don't carry over), and cheat them back?
