Instructions
Sign out of all the profiles you saved on your Xbox 360. Press the Xbox guide button located in the middle of the controller and select the option, "Recover Gamertag."
Follow the directions onscreen. You will be asked for your name, address and password used to set up your Gamertag.
Use the Xbox 360 controller, Chatpad or keyboard to enter your information into the Gamertag recovery system. Once completed, your Xbox Live profile will be downloaded to your console where you can now log-in.