You can edit and export (share out) an iMovie project to a movie as many times as you want. Of course, if you have deleted the project that feature would not be available. In that case you could still import the exported movie back into a newly created iMovie project and edit it to a limited extent, such as splitting in segments, deleting segments, adding titles, adding effects, adding or deleting music, importing new clips, etc. You just couldn't undo any embedded edits that were done previously in iMovie.
I can't think of any reason why you could not find your original project unless you deleted it. It should be located in the projects browser (where your projects are displayed as icons). But, in any event, you can retrieve your project 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
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.
Another way would be to go to your Time Machine backup, if you have one, and restore your library to a point in time before it disappeared.
-- Rich