Did this affect any of your current iMovie projects? I'm having the same problem, but I don't want to mess up any of the projects I'm currently working on. That would be hours of work to redo.
Also, is it possible the last iMovie update caused this problem? I ask because I'm having the same problem on two different Macs, and the last time I remember it all working okay was back in April. Hasn't there been an update since that time?