iPhone 7 is frozen and I can't turn it off
I would repeat this process two times 5 minutes apart and one last time 15 minutes later and them move on to the more painful remedies.


https://support.apple.com/en-us/HT201559
https://support.apple.com/en-us/HT201412 (try the force reset only unless you want to escalate to restores)


Once you’ve exhaused asking nicely and not so nicely for the device to redstart, you have options that require a computer or more time:


Wait for the battery to die / exhaust itself. Then plug it in and see if it works or needs to be restored.
Call the phone / text the phone / try to get something to work that you can accept from the lock screen to know it’s still basically functioning. Sometimes after answering the call, you can then get to the unlocked device - other times, you’re back at the lock screen.
Connect to iTunes and restore the device.