Hi,
You might be able to retrieve your past work from the iMovie Backups folder on your Mac. To get there, 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 that is dated before you had the random deletions issue. iMovie will open in that library. Navigate to your project and see if it is intact without deletions.
Another thing you can try is to use your Time Machine backup to restore your iMovie library to an earlier time before the deletions started to manifest. Restore it to a different location so that you don't overwrite your old library. The restored library would not contain any work that you may have done subsequent to the date of the restored library.
Hopefully you will be able to recover your past work. However, you need to address the issue of random deletions because it might occur again even if you have recovered your past work. Possibly there is some corruption that is confined to just this one
project. You could try placing your media into a newly created project and see if the random deletions stop occurring. Or maybe you have some corruption in your library, in which case you can try opening iMovie in a new library and see if you can create a successful project there. Or, you could try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to
get back to your projects. Now see if the random deletions are cured.
If the problem keeps occurring, you might consider contacting Apple Support. They are savvy and have testing equipment where they can access your computer, with your permission. To contact Support click on the Contact Support item in the upper right hand corner of this forum's screen. Navigate to a telephone number where you can call Support.
-- Rich