Try the procedures listed in this Apple bulletin:
https://support.apple.com/en-us/HT211916
If still no luck, try these procedures:
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 from System Preferences/Users and Groups and try to open and use iMovie in the new account.
You can use the Guest account on your Mac for this purpose.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads,
immediately shut down and reboot in normal mode. Then see if iMovie works properly.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
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 Get Support item in the upper right hand corner of this forum’s screen. Navigate to a place where you can enter your phone number.
-- Rich
