inactive message on display

Hello Marco, have you found a resolution to this problem? Beyond waiting for 15 minutes to see if it worked, did anything come of it. What happened to the phone in the days leading up to this issue?

You could open up the device and take a good look at all the connectors and flexes. Look for partial disconnections or torn flexes. Disconnect the battery and wait for a few minutes before reconnecting to see if that resets the phone properly. Finally, try performing a hard reset (hold the Home Button and Power Button for 10 secs) and if data retention is not issue, try doing a DFU restore.