Hi,
Deleting preferences is a harmless procedure that, in any event, affects only the iMovie application and not the libraries that contain you projects. You projects would not disappear unless your libraries were previously corrupted (which certainly might be the case in light of the issues you have been experiencing).
To get back your projects go to the iMovie Backups folder. To get there, follow this file path: Go/Home/Library/Caches/iMovie Backups. (If you don't see "Library" after clicking on Home, hold down the Option key while clicking.) In the Backups folder you will see a list of backups of your libraries. Open the library dated just before you started having issues. iMovie will open in that library. You should be able to retrieve all of your projects in the Projects browser view, as long as the original media still exists.
Another way is to restore your library from your Time Machine backup, if you have one.
-- Rich