Cannot power on iPhone 8
I have a new att iphone8 . Window popped up with message that my phone has not been backed up on iCloud for 2 weeks With instructions to connect to wi fi , power source and enter passcode. I did and then instantly the screen went black. I read online that first time might take 2 hrs approx. I left it overnight and it still won’t power on. Please help. what can I do to restart.
I can’t get serial number because I cannot power on!
thanks for your kind attention.
eileen
iPhone 8, Att phone and service