I movie ...

Make sure that your iMovie app is updated to current version.

Make sure that you are opening iMovie in the library in which your projects exist.

When you open IMovie, go to the projects browser view. If your projects don't show there, go to the top of the project browser window and look at the selection box to make sure that you are in the correct library. Toggle with the selection triangles.

-- Rich

If you keep a Time Machine backup you can use it to restore your iMovie library at a point in time before the projects were lost.

Also, you should look in the iMovie Backups folder to see if you can open iMovie in a previously backed up iMovie library and access your projects there.

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