Frozen phone

My son plugged his IPhone7 into his PC laptop to charge the phone, and the phone now has a dialog box that says "ALLOW THIS DEVICE TO ACCESS PHOTOS AND VIDEOS? This device will be able to access photos and videos while it is connected to your I Phone. ALLOW / DON'T ALLOW". He then unplugged the phone and this dialog box does not disappear, does not respond to his touch, and he cannot do anything with the phone. The touch screen is frozen. He has tried to force a shut down with the side button and down volume button but without success. Any suggestions as to how to shut down or restart the phone or otherwise get out of this frozen state?