Cannot Play or Preview Movies

I imported several videos last night and was looking to organize the clips
into one long collage. I got a good bit done but when I woke up this morning
and opened Windows Movie Maker, none of the clips would play in the preview
window. It says 'Playing', but the counter remains at 0, the screen remains
white (or green sometimes), and there is no video or audio. If I add the
clips to the timeline, I get the same problem.

The clips were working fine right after I imported them, but after I
re-opened Movie Maker, none of them worked at all, and they don't even work
if I re-import the same clips.

If anyone could help me, it would be appreciated. Thanks.