Some info on your system would be helpful. Is it on your Macbook? What OS are you running? Is iMovie up to date?
Whenever you get an app that crashes, first repair permissions. If that doesn't help, your imovie preference file may be corrupt (.plist) simply trash/empty trash and try again.
Also, have you tried to start another project? If that's ok, maybe something is bad in your current project. You may have to bring in your media into a new project if the above doesn't help.