Is TV-out an OpenGL feature ? Not really.
Anyway some basic recommendations :
TV-out chips can die. I had one card, in which everything worked well, then suddenly no tv, and visual inspection showed a burned component. Everything else was still ok.
maybe tv cable is not plugged correctly, or tv is off, and driver only allow tv out when a tv is detected at the other end.
just before it break, did you changed drivers or anything ? played with rivatuner or this sort of stuff ?