HI, I made a thread about the video preview in Vegas not working. The audio is fine but the video doesn't show, the preview screen is just black

It says:

Video: Stream attributes could not be determined
Audio: 128 kBit/s. 48,000 hz. Stereo; 00:23:14;17; MPEG Layer-3

I am really sorry for nagging, but I desperately need help!