I have not had the same problem you have had, but I have had the Sims lock itself in a similar manner to the way you describe. I use a computer with two monitors, so I just click something in the other window, then jump back to the Sims and it seems to correct itself.
If the game's not frozen, have you tried just minimising it (Windows+D) and then clicking the taskbar to bring it back up again? The way you describe it seems like a loss of window focus. Maybe it'll work, but it sounds like a curly problem. Good luck.
I'm assuming you use Windows because you mentioned Task Manager.