My Wii won't show video for NES or SNES games, can you help?

I turn on my Wii one day, on my nephew's request to play Legend of Zelda a Link to the Past. When I select the game, I heard the sounds (The Nintendo chime, the sound of the three Triforces joining into the logo), but there was no video. Is there some problem with the cables, or do I have a bigger problem? I would hate to have to reset me Wii because I have a bunch of pokemon saved onto my ranch on the game.