If your not very far into the project, I would strongly recommend simply copying over the data from the old project to the new project, and that should cure the issue.
Most likely, the project file itself corrupted.. ( this happens infrequently, but does happen now and then )
If you have too much time invested into the corrupted project file it may be possible to recover it.. But if it would take less then half an hour to copy the data to a new file, that would take less time then our efforts to repair it.. let me know which way youd like to proceed..
That being said, whatever route you decide on, we definitely want to take these two repair steps just for good measure.
Open your finder ( magnifying glass ) type in disk utility, and launch the disk utility application.
On the left side of the disk utility window, click your mac hd, then at the lower right hand side, click repair permissions .
And also, you want to follow the steps at the bottom of the link below, to trash the imovie and iapps plist files..
Those steps combined, should correct any corruption of your mac files and imovie files that may have caused this issue.
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Senior Technician and Network Switching Systems Operator / Maintainer.