You'll have to factory reset your console. All the gamertags should be backed up so they will be able to be redownloaded. Back up all your saves to the Cloud first.
take down the serial # off the back of the console (how to find it - support.xbox.com/.../serial-number)
Then call Support - support.xbox.com/.../contact-us (click Request a Call).
They'll give you a set of controller instructions (i.e. up up down down left right left right B A) unique to your serial # that will allow you to reset your console.