Hi, Eliot,
Yes, try the deleting preferences process and see if that cures it.
If still no luck, try opening a previous automatic backup of your library contained 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. Opening a previous backup of your library will not overwrite your original library.
If still no luck, import your successfuly shared movie into a newly created iMovie project. It will come in as one large clip. You can do some limited edits, such as Modify/Split Clip to create segments that can be independently edited, modified, or deleted. You can add different music and, in some cases, replace titles. You can add different clips. The only thing that you cannot do directly is to change edits that were embedded into the original cut.
-- Rich