Today my household disappeared too, but this is probably a slightly different case than described above:
I switched temporarily from playing my main household to a different one. Meanwhile I also decided to "clean up" my unplayed households and deleted all randomly generated sims as well as a joined household which contained:
- a (different!) copy of sim A who I also played as my main household (saved separately) and
- a copy of sim B who was also saved as a separate unplayed household.
So the deleted household was a superfluous household, as both sims were saved as separate households (at least so I thought)!
But when I tried to go back to my main household (copy of sim A) it was empty and not loading. Also the second household (copy of sim B) was empty. Both households are still listed in played or unplayed households, respectively, but without sims.
It appears to me that somehow the joined household and the separate households were connected. Otherwise this would be a big coincidence.
I also tried the "resetsim" but the sim isn't found anymore...
*Update: I'm not sure if the joined household was listed among played or unplayed households, but it contained definetely a separate copy of the sims.