How to restart stuck Nokia Lumia 720 with damage volume down button? Is it possible to restart without volume down button?
On my Lumia 1520, simply pressing and holding the power button for about 10 seconds will hard-reboot the phone.
You can also try some things that may unfreeze the screen, like plugging the phone in or connecting or removing headphones, or holding the power button for a few seconds (like a normal shutdown) and seeing if the lock screen slides down from the top. I've had most of those work at one time or another, even when the phone was otherwise unresponsive.