I had the problem once, and it was as Roo suggested.
Except this was in a PC. I used to have it underneath the desk, which happened to be right between my legs. And sometimes if I had the chair too close to the computer, and I turned the chair around it would knock it over.
Anyway, one time apparently it knocked it over and loosened up the vid card.
I didn't get a green color, I would get "little glitches" (at lack of no better words)
Example: In windows, pieces of my desktop and icons would be fuzzed out.
After taking out and reseating the vid card. Everything worked fine.
I have since moved the computer on top of the desk, so I can't knock it over anymore.
