how to correct a frozen screen and phone won't turn off

If your iPhone is frozen, try doing a Forced Restart:

1. Press and quickly release Volume Up

2. Press and quickly release Volume Down

3. Press and hold the Side Button, the phone will restart. Release when you see the Apple Logo.

You can find more useful information here:

If your iPhone won't turn on or is frozen - Apple Support

1. Restart the device. That's it.
2. Still not solved --> Force restart iPhone - Apple Support

1. Follow the Apple article to resolve the issue --> If your iPhone, iPad, or iPod touch won't turn on or is frozen ...
2. If not resolved; please follow the steps in the following link for support with this issue: If your iOS device restarts or displays the Apple logo or a spinning gear unexpectedly

• You still see a black screen after charging, force restarting, following the steps in this article.
• You see a black screen, but you can hear alerts, sounds, and vibrations.
• The screen remains black, but the device chirps when you unmute it and connect it to power.
• Your screen is on, but it doesn't respond when you tap, slide, or try other gestures. 
• You can't force your device to restart because a button is broken or stuck.
• Your device is stuck on the Apple logo, displays a solid color, or isn't recognized in iTunes.

Contact Apple Support to set up service if you have any of the above issues:

Try a "forced" restart of your device using these instructions: Put it on charge for at least half an hour, then, keeping it plugged in, use these gestures:

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).

Regards

Giulio