If you still can see something on the screen, enable Assistive Touch in Settings - General - Accessibility. You'll see a semi-transparent dot somewhere on your screen. Tap it, select "Device". You'll see "Lock Screen" as one of the options. Tap and hold it until you see "Slide to power off". This way you can turn it off.
If the screen isn't working any more, you'll have to wait until it's turning off by itself because of low battery.