Hi,
Try deleting/resetting preferences and see if that cures it. 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. 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.
You might also try creating a new project and see if that will resolve it.
Also, check to make sure that you have sufficient free space on your internal drive for iMovie to operate.
-- Rich