When you alt-tab to show the processes, is task manager listed?
If yes, then it is hiding off the screen, if not then...
something more serious
Regards,
Shuttlebus
P.S. Just an idea (since it is a long time since I had this problem (Win95.... don't ask). Have you tried dropping your resolution to something truly awful, like 640x480, selecting task manager using alt-tab (assuming it appeared in the solution above using alt-tab) and then boosting your screen res back to something vaguely useful (1024*768 or 1152*864)? If that doesn't work trying going from large to small resolution instead.
Also, does you graphics driver support "virtual desktops"? Some of the Matrox drivers allow you to have a virtual desktop of say 1600*1200 whilst the monitor is running say at 1024*768. Sometimes if you then disable this feature with something in the virtual space, it gets "lost".
OK, that is my brain well and truly picked....