When I tried to save my game a while ago, I found that the controls for Save, Neighbourhood, and Quit were all disabled.
My guess is that this happened because they family's daughter (who's now in college), while visiting her parents' household, answered her cellphone and accepted the invitation downtown, then took off in a taxi -- and that the game is now waiting for her to come back, which she won't since I can't switch to her.
I found this entry which looked a bit similar to my problem (though I think Build and Buy mode worked for me), but since the solutions involved objects/mods I didn't have, I decided to kill the game from Task Manager and will have to play them again next time I start the game. Ah well.
(And her poor old dad will die all over again, I suppose, since he died during this recent session while she was there.)