Hi, miya82,
In order for iMovie to link to the Photos app the Photos app library must be designated as the system photo library in the Photos app General preferences. The words "Use as system Photo library" must be grayed out. So, check that.
To shut down iMovie when it appears to be non-responsive, in the Finder menu go to Apple Logo/Force Quit/iMovie.
Once you get iMovie open again, see if it will link to the photos app and close in the normal fashion. If still no luck, try deleting preferences. To delete 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 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.
-- Rich