New iphone12 - Can't use

Hi, I recently upgraded my mobile to iPhone 12pro. Received it last week and was working great up until two days ago.

I can unlock the phone but can't use apps including calls and messages. Can't answer calls either. I've taken SIM card out, connected to computer, I can't figure out how to turn it off either. If I do manage to click on to an app, the screen stays dark and doesn't bring it up. Help.