Hi,
Assuming that your computer is a stand-a-lone and not on a network system, try deleting/resetting preferences. That procedure often resolves issues when iMovie is not functioning correctly.
To delete/reset 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. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon. iMovie will open in that 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.
Another thing you can try is to create a new project and see if iMovie functions properly in the new one.
Also, be sure that your iMovie app is fully up to date.
Finally, there is always the old standby of simply restarting the computer.
Good luck with your project.
-- Rich