Before moving projects or libraries, it is good practice first to do a File/Consolidate Project Media in the case of moving a project, and a File/Consolidate Library media.in the case of moving a library. That will ensure that all of the media transports with the project or library. You might try those procedures now and see if that restores the link to the media.
Possibly moving the project back to its original location in the originating library might restore the link on its own. Then you can move it back after doing a File/Consolidate Project Media.
Another possibility is to restore the project with Time Machine to an earlier point in time before you made the move.
Here's another thing that might work. iMovie automatically backs up the iMovie library that is in use and saves the version to the iMovie Backups folder on your hard drive. So, you might look in the iMovie Backups folder to see if you can open iMovie in an earlier saved version of your library and retrieve your project from there with the links intact.
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 project is there with its links intact.
-- Rich
