Check to make sure that you have sufficient application space on your drive for iMovie to open. Sometimes our drives can fill up without us realizing it. You would want about 10GB on the drive.
Also try updating iMovie to current version, since you have updated your OS to Monterey 12.5.
To see if there may be some corruption in your present iMovie library, try deleting preferences as that will cause iMovie to open in a new blank library. 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 blank library. Reopen your old library to get back to your projects. 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.
If iMovie still won't open, try going through the following diagnostics to see if you can cure the problem.
Shut down and reboot
Try shutting down and rebooting, as sometimes that will will cure some issues.
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 will load.
Start up your Mac in safe mode - Apple Support
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.
Set up users, guests, and groups on Mac - Apple Support
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
Reset the SMC of your Mac - Apple Support
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM:
Reset NVRAM on your Mac - 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 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