If still having trouble, make sure that your iMovie app is updated to current version. Sometimes that cures odd problems like you are experiencing.
Another thing that you can try is to open a backup of your iMovie from 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 or later,
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.
-- Rich