In reply to mypyrex:
It sounds like the application is positioned off the edge of your screen.
RIGHT click on the item on the start bar, and choose move, you should then be able to relocate it using the arrow keys. Alternatively right click on the start bar (away from the icons) and choose show windows stacked, or show side by side, and it should come back.
Have you been using the laptop with an external monitor or extended desktop? That can cause this if the machine dpesnt register the second screen being disconnected.
Hope this helps.
P.s. this problem is just as likely on xp and win7