Unfortunately, I think you are pretty much stuck, unless you keep a Time Machine backup. If you have a Time Machine backup you can restore your iMovie library to a point in time earlier than when you made the deletions. Or possibly you have some other type of backup on an external drive that you can use.
iMovie does make automatic backups of the iMovie library as you do your editing. These are kept in the iMovie Backups folder. From the backups folder you can open previous backups of the library. However, if the media itself as been deleted, as in your case, I'm pretty sure that the iMovie library backup will also be corrupted.
On the outside chance that the iMovie backups folder contains a functioning backup of your library with the projects intact, you can try accessing the folder. Here's the way to do it:
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 deletions were made. Hopefully, iMovie will open in that library and the library will be functional. If it opens, navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact.
The lesson to be learned here is to never, ever, delete anything from the Show Package Contents folder of an iMovie library or, as happened in your case, you risk corrupting the library and its projects.
The only other option that I can think of is to purchase some recovery software and see if you can recover the files that you lost. Otherwise you will need to re-import the media and rebuild the projects.
Of course, any project that you had previously exported to a standalone movie file could at least partially be salvaged by importing the standalone movie into a newly created iMovie project and editing it there. Editing would be limited, but you could still do quite a lot, such as splitting the clip and deleting or editing the segments, adding new clips and music, changing the music, in some cases adding titles, adding effects. The only thing that you couldn't do would be to change any embedded edits and effects that you had done previously. You would need to cut those out and re-import the affected video clip.
Sorry you are having this trouble.
-- Rich
