Hi,
Here are a few more things that you can try.
Opening iMovie in a new library
Try opening iMovie in a new library. To do that, open iMovie while holding down the Option key and then selecting to create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account and try to open and use iMovie in the new account. You can use the Guest account on your Mac for this purpose.
Change Users & Groups settings on Mac - Apple Support
New Project
Another thing you can try is to copy your media into a newly created project and see if that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Start up your Mac in safe mode - Apple Support
Contact Apple Support
If none of the above resolve your issue, contact Apple Support. The techs there have the capability of remotely accessing your computer that we cannot do here. To contact apple support click on the Support item above this forum’s screen. Navigate to a place where you can enter your phone number.
-- Rich