It could be the video card. Go into the options (Big R -> Options) and on the Graphics tab turn the Hardware Acceleration off (uncheck the box). Then restart Revit and see if the Ribbon is back.
I had the same problem, although I think it was because of different reasons. I was using a 32-bit WinXP computer to connect via Remote Desktop to a 64-bit Win7 computer, and the ribbon was "invisible" but still active. Some people suggested that I shrink the size of the Revit window (instead of being full-screen), but it didn't work for me. Maybe it'll work for you, if my first suggestion didn't do the trick.