OKAY, I found this thread when this same thing happened to me and just found a very surreal way to fix it:
For context, I'm doing a "perfect sim" challenge, and had played the sim from when she was a toddler living with her grandfather. The grandfather had just died and my main Sim was the only one in the household when I tried to move out. I quit the game and came back in to find the same blank household that everyone else did.
I didn't want to completely give up on it, so I decided to reload the original household back in and cheat my sim back to roughly where she was. So my toddler and the grandfather load into the house (exactly as it was when I tried to move), and the older version is just standing behind them. Not in the household. Just...standing there.
I immediately paused the game, and then added her into the household. She was exactly the same. She had all of her relationships, all of her inventory items, bonus traits, and skills, and she was enrolled in University.
I deleted the young version of the household, and now I'm just going to move her into an apartment or something and go from there.