I would recommend first clearing your system cache:

http://support.xbox.com/en-US/xbox-360/settings-and-initial-setup/clear-system-c ache

Then you'll want to make sure that the necessary ports are open:

http://portforward.com/networking/staticip-xbox360.htm How to set up a static IP address on your Xbox 360.

http://www.portforward.com On this webpage click on Router List > Select your router > Select your game

Disable any firewall or security features on your router.

Set port forwarding on your router to the IP address of your Xbox 360. This game uses port 3074 (UDP/TCP).

Additionally Xbox Live requires ports 80, 53 (TCP) and 88, 53 (UDP).