How do i get all the video clips to show when editing?

I am trying my first project.  I have a number of video clips that I don't want on the same video line.  I want them on different tracks (which they are) and have them all line up at the desired spot and time to the seperate Audio track.  When I hit play there is only a black screen.  I can see the time line moving and hear the audio track but dont' see any of the video tracks.  They are not hidden, they just don't show up so I can't play them to line up against the audio track.  How do I fix this?  This is maddening!