Might be that the library has become corrupted. Try looking in the iMovie backups folder for a previously saved version of your library and see if that one will open.
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, 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. See if iMovie will open in that library. If so, navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact.
-- Rich