Instructions
Turn on your Xbox 360 and wait for the main menu, or dashboard, to load. If you are using the external wireless network adapter with your Xbox 360, unplug it from the console's USB port.
Select "My Xbox" and "System Settings" using the controller's left analog stick. Then press the "A" button. Select "Network Settings" then "Wired Network" (if asked to do so) and press "A" again.
Select "Configure Network," and press "A" on the controller. Choose "Additional Settings," and then select "Restore to Factory Defaults." Press "A." The console will reset the DNS addresses and other network settings associated with your Xbox 360.
Turn the Xbox 360 off and reconnect the wireless network adapter to a USB port if necessary. Turn on the Xbox 360 and return to the "System Settings" menu on the "My Xbox" dashboard tab. Select "Network Settings" and "Wired Network" (if asked), and then select "Test Xbox Live Connection," and press "A." Your Xbox 360 should connect to the Xbox Live servers.