This usually happens if you have moved or renamed an Event folder or an Event clip from where it was when you created the Project. If you can move things back where they were, it should work again.
If you shared a copy to iTunes or to the Media Browser, it may still play, because there would be a copy of the finished movie inside the project package. To see if this is the case, you can right-click on the Project in the Finder. Choose "Show Package Contents" and look in the Movies folder. Be careful not to change anything inside the Project folder or you can maybe corrupt the project.