I had this happen the first day I played. I can offer advice on what NOT to do. Don't save your game if they get stuck. You will lose that sim and their entire home town. I learned it the hard way.
You guys can trade for some of my roommates. They're constantly in the shower or tub so my sim can't get in, and they cook more food than they'd need for six houses. Also, the fact that no one ever uses the hampers makes me crazy.
In a manner of experimentation, I left the University screen and went to the main menu. When I went back into my game, the entire town I'd been playing was gone, the name of the stuck sim was being asked where she wanted to live on campus (as though she was just arriving), and the sim herself was completely gone from the…
I just had this happen too. My first graduate went home just fine, but the second one is stuck. I can do day jobs and things, but I can't click on the phone.