Phone’s screen has suddenly turned blue and cannot be turned on or off

My phone’s screen has suddenly turned completely blue. It cannot be turned on or off. I bought the phone through giffgaff. The phone is under warranty so I can send back for repair however giffgaff say I must turn off ‘Find my phone’ before they can receive the handset to repair. But when I go to iCloud to do this it sends the Verification code to my phone which obviously I cannot use as it’s out of use. I’m going around in circles. Can anyone help? 🙏🏼

[Re-Titled by Moderator]