Halo v1.0.10 and the Gamespy shutdown
Update your Halo to version 1.0.10
The company (GameSpy) that provided the Internet Lobby has shut down. Thankfully Bungie has published a fix for the Lobby!
If you did not receive the in-game update before July 26, 2014 you can still manually update your Halo, details on how to update are available at Halo Fixes.