Hi, I have the exact same problem it is cause by the date and year being changed to
years earlier my duaghter did it. It can be fixed but if you don't have all of your data back
up you will lose it all. Ring Apple tech suport they will talk you through it.
Just have ready the serial number, your password, email and unlock number. Also you will need to be loged into itunes or they can't do it.
I hope this helps you.
Regards
Anthony