Why does my sega gensis get sound but no picture?

I recently took my console over to my girlfriend's house and tried plugging it into her cable box with the regular RF cable and then realized this was not going to work. I brought it back home and when trying a new game, I would get sound but no video, when before this trip it was working. I thought I was very careful with it and I have tried blowing out the game port but that hasn't worked. I have tried multiple games with the same problem of sound but no video. I want to play maximum carnage...help please?