Once I knew what and when it happened, I was able to avoid it happening again and was able to go on. I also use MCCC which shows a wall post when an LE occurs and that let me figure out exactly when the LE occurred and gave me the specific error that caused the LE. After recovering the save, I was able to duplicate the event several times.
In trouble shooting what happened, I was fortunate as I was able to use a recovered save to get them back so you might want to see if you can recover a save where your sim is still in the game. Both times it has happened, I had a Last Exception (LE) at some point during my play prior to the save and exit. I have had a couple instances recently where the active household has completely disappeared on me when reloading after exiting and saving.
