It seems that either the iMovie app is not functioning properly or there is some glitch in the specific project.
You might try the following:
- First, do an Edit/Undo as many times as necessary to see if you can get back to where you were.
- Failing that, delete preferences by opening iMovie while holding down the Option and Command keys, and selecting to delete preferences from the box that appears. iMovie will open in a new library. Reopen your original library to get back to your projects.
- If still not functioning correctly, transfer your media into a newly created project. To do that, do an Edit/Select All, Edit/ Copy, and then Edit/Paste the media into the timeline of a newly created project. Your edits will carry over. And if they don't you still have your old project intact.
- If still no luck getting iMovie to work correctly, try reverting to an earlier backup of your iMovie library in the iMovie Backups folder. To get to the iMovie Backups folder, follow this file path from the Finder menu:
Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups
NOTE: If running Ventura or later, in the above
file path substitute iMovie for com.apple.iMovieApp. So, for Ventura,
the file path would read:
Go/Home/Library/Containers/iMovie/Data/Library/Caches/iMovieBackups
When you get there you will see a list of previous backups of your iMovie library. Click on one dated just before your issue started. iMovie will open in that library. Navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact.
5.As a last resort, if you keep a Time Machine backup you can restore your library to an earlier point in time from when you made the volume change. Restore it to a different location so it does not overwrite the original library.
-- Rich