Windows Media Player plays videos all in black?

Can anyone help me? I was watching videos I had recorded on my camera, renaming the videos on the computer as I went along, after watching one video, immediately when I clicked the next one, everything was in a pitch black, but the audio is still there, I played other videos using windows media player & the same thing happened, can anyone help me??? Even on other players like quicktime player, the video appears screwed up but the audio is working fine, what happened?? Can anyone help??