So, it seems like I completely broke my Sim, somehow. I loaded up my neighbourhood and I noticed in the household view, that my Sim was at the bottom of his aspiration meter. I went into the household and saw that he had his aspiration switched from Fortune to Family, and now he was in deep red. A few minutes later I looked at his Simology and saw that everything was gone. Memories, personality points, interests, skills were also missing. I also noticed his fitness bar was pushed all the way to full, but the game still recognised him as thin. The last thing I did before he was wrecked was go into SimPE and changed his and 4 other Sims names, yet he was the only one to lose everything (everyone else is fine). Right before I changed all the Sims names I backed up the neighbourhood out of fear of breaking something, will restoring the back-up fix these problems?

Yes, restoring from the backup should fix it.

Thank you, I will try this!

EDIT: Everything is back! Next time I'll try be more careful when messing around in SimPE...