This happened to me just now, only it got progressively worse until my Sim didn't appear on the main menu screen at all (I only controlled one, so that was my whole household in effect), and when I went to play after I restarted my computer, the game menu (with all the skills, mood, EVERYTHING) disappeared. I can see other Sims getting on with their lives in the martial arts school (where my Sim was last seen) but can't do anything. My mouse can't interact with ANYTHING on the screen. Tried the resetSim gag as well, but it seems to have forgotten that my Sim existed.
Super **ed off, because I got her really far. This happened whilst she was travelling to China, but she lives in Bridgeport.
If anyone knows how I can possibly fix this, let me know.