A/V problem?

for some reason, within the past 5 months of me not playing my dreamcast, when i tried to play it last night, i could hear sound but see no video on the screen…games loaded perfectly fine and everything, i loaded nfl2k, and heard the commentating and everything from the game, so i know the dreamcast works, but for some reason i see no picture, i tried multiple cables, but still no picture, what could be the problem?