you have moved the video file to another location. move it back, or re-link to it's new location.
if thats not it, were you editing from an external hd? plug it back in. (though i don't suggest actually editing fom one.
KioAtWork: I'm so bored. I don't have class again for another half hour.
Minion: masturbate into someones desk and giggle about it for the remaining 28 minutes