The first thing is to play the old swap around.. There are, Depending on how it's wired, 3 or many components in your hook up
DVD player (not to mention the DVD itself) The cable to the TV, and the TV. Optional are cables to switches, RF Modulators and more switches. (this cn get downright complex) My Video hookups here look like something the rats nested in with all the connections to the modulator/video switch, home theater, and more. That said.. I can follow every line if I have to.
Start by using a portable TV with direct video in.. Do you have video if you simply transfer the video (yellow) cable to it? If so then it's the TV, if not try a dfferent video cable.
now try a differnet video source in place of the DVD player, Considering you can pick up a DVD player at a pawn shop for around 10 bucks... That would by the easiest (less you have an alternative)
If you have a full home theater like I do.. Then it gets a tad more complex.