Yes, that definitely is a bummer. Nothing to do but try some diagnostics and see if you can shake out the problem. Here are some things that you can try.
- Since you've talked with Apple support, I assume that you have verified that you have sufficient space on your application drive for iMovie to operate. 10 GB would probably work, but 20 GB is better.
2.In case there is some conflict that is causing the problem, try rebooting your Mac in Safe Mode and, once everything loads, shut down and boot up in normal mode. Here's a link with some instructions:
Start up your Mac in safe mode - Apple Support
3.If still no luck, try deleting/resetting preferences. To reset/delete preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (Before reopening your old library, see if iMovie functions properly in the new library.)Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
4.Failing that, if you have an intel Mac (versus one of the newer M1 or M2 silicon Macs) try resetting the System Management Controller. See the link below:
Reset the SMC of your Mac - Apple Support
5.Should the problem persist at this point, you might see if you can open iMovie from a previous back-up of your iMovie library. 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
NOTE: If running Ventura, in the above file path substitute iMovie for com.apple.iMovieApp. So, for Ventura,
the file path would read:
Go/Home/Library/Containers/iMovie/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.
6.Another thing: If you keep a Time Machine backup you might try restoring your iMovie library to an earlier date and see if iMovie will open there. Restore it to a different location than your present library so you don't overwrite it. If you restore to an earlier date, you most recent edit will not be there.
-- Rich