I've had that a few times too... unfortunatly it means your game didn't manage to save all the necessary data. Last time I checked SI are not able to fix your save-file or tell you how to do it (as it's probably far far too complicated) So I'm afraid you're going to have to start over again.
I'm constantly saving between 3 different save-files, it usually makes me able to start my
game again if I slip up, a little in the past, but at least I don't have to start all over again.
Sorry mate, that one is toast