iMovie 11 will not recognize iSight Camera (built-in)

I have record dozens of iMovies before using iMovie 11 from the built-in camera on my Macbook Air. All of a sudden when I go to record another one, it says "No Camera Connected. To import video, please select a camera." I have no options when I go to select a camera and there is nothing I can do. I have tried everything and this is extremely urgent! Please help and thank you for your time!