won't play any clips but first

[nicoletti07]

I have vista and have used WMM for the last couple years with no problem. I finished a movie about a month ago and now went to add another video. After creating my clips, I went to play them back. Only the first clip will play. The other clips just show a black screen and don't move. Additionally, if I pause the first clip while it's playing in the preview screen, it will not restart. Or if I try to drag it ahead. It totally freezes up. Nothing has been added or done different to my computer. I tried restarting the computer, thinking it was just a glitch. I even deleted the video and re-made the clips. It is doing it with movies that I previously created clips for long ago as well. PLEASE HELP! I'm at a total loss.