Black screen iPhone 12 Pro Max

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2)  If the above does not work then - Follow the instructions in this article by Apple ➔ If your iPhone is Non-Responsive or Frozen and see if that helps.

Axel F.

If a forced restart does not help after the phone cooled down again, visit an Authorized Apple Support Provider to get it diagnosed. Or contact Apple Support to send it in for service.

Force restart iPhone - Apple Support

Apple - Find Locations

iPhone Service & Repair - Apple Support

Did you charge it by using the Apple charger and the USB cable?

If you visit an Authorized Apple Service Provider next to your place, show them this accessory as well, to get it diagnosed.