Projects cannot be opened from the Show Package Contents view. The projects there are only data and codes and cannot be played except through the iMovie app itself. As to why the projects disappeared, it's hard to tell.. Possibly the link got broken between the iMovie app and the library.
On the other hand the projects may not have disappeared at all. Make sure that you have opened iMovie in the correct library, the one that contains the projects.
If all O.K. there, try looking 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
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