Instructions
Press the silver guide button on your controller. Wait for a menu to appear on your television screen.
Press "Down" on the left analog stick until you highlight "Recover Gamertag." Press "A."
Type in the login information for the profile you wish to recover. This means you must type in the email address and password associated with your account.
Wait for the console to recover the gamertag. This may take several minutes. Once complete, the console reboots and you can log in to your profile again.