You probably should redownload iMovie. Drag your iMovie app from the Applications folder to the trash, but do not empty the trash. Then sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You purchases will be displayed, including iMovie. Download it from there. If no success, drag your old iMovie app out of the trash and put it back in the Applications folder.
In the complex electronics world glitches inexplicably can happen. Here's a trouble shooting link that might help you isolate the problem:
https://support.apple.com/en-us/HT206157
All else failing, you might benefit from calling Apple Support. The techs there are pretty savvy. Click on the Contact Support item in the upper right hand corner of this forum's screen and navigate to a phone number. Hopefully Support will have an answer for you.
-- Rich