|Have they ever played before on this computer?|
In any case, you should reinstall codecs and check if it works.
I'd try two things in that order:
- Install VLC Player. it's free and much more trustworthy than Windows Media Player!
Try your video with VLC. If it works, and you really want to use WMP, try updating WMP.
- Install ffdshow. If MPEG is not playing, you probably don't have the right codec to play it. ffdshow is a media decoder and encoder that includes pretty much every common codecs and will be automatically used by apps like WMP without anything to do but install it.
It should help.