After IOS 12 upgrade the screen on my iphone 7plus was completely frozen. Resetting phone a number of times produced zero results. The only thing that worked for me was to put the phone into DFU mode (Hold power and Volume down button simultaneously untill DFU mode is entered). Then plug phone into Itunes and click restore button but only allow restore process to continue for a few seconds. shut down itunes, unplug cable to phone and hold power and volume button until logo appears. Phone should now work.