I actually have this problem soon after I upgraded to iOS 11, the touch is not responding at all. After a few trials, I was able to switch off the phone successfully. Here is what you should you. Open iTunes on your laptop or Mac then connect your phone via USB and then press Home button and Power button simultaneously until the phone gets into the restoring mode. As the phone enters in restoring mode, disconnect the USB cable and then simply hold the power button to switch it off.