The only way to factory format the phone is through the Recovery mode. To access recovery mode, press and hold down Volume UP + Home + Power buttons at the same time. Release the buttons when the Android logo appears then use Volume buttons to select Wipe Data / Factory reset and then reboot the phone. The system will be initialized and you'll be able to use the phone. If the display still doesn't respond, you will have to take the phone to a Sony support shop because the display might need to be replaced.