Why wont my Sony Vegas Pro. 8.0 let me see the picture of videos I put in?

I've been using this Sony Vegas Program for 2 years and its been working very well. Now all of a sudden if i drag a video into it the video files come up as just a black picture.. but the sound comes in. It never used to do that. And I'm using the same type of video file I have always used. What could the problem be??