Xbox

Every time I try to to xbox live this screen comes out with error 21. What does this mean

 

go here it explains the error messages
http://www.xbox-scene.com/articles/errorcodes.php

21 - anywhere - This error says that the machine was booted to display a error, basically someone told the machine to reboot (or launch a xbe) with this flag, and the error code just means its been rebooted by the flag