Sounds like it could be a video card issue or some video processing issue. We get this problem every-so-often at the school I teach at. Manipulators don't show up, you cant see keyframes in the timeline, and certain tools don't work correctly. I've found the common fix for that is to simpy reboot the PC and it shows up. It's a weird thing and nobody seems to know why it happens. Give that a try and let me know if it works.
=D