It appears that some players have been having issues connecting to the Halo 4 servers to play recently. The issue is thought to be connected to the most recent release of DLC for the game that came available Monday. 343 Industries is aware of the problem and are currently working on a solution.
“There are a few hundred players who have been unable to connect to our servers since yesterday,” stated a member of the Halo Waypoint team. “We’re investigating now and hope to have an update soon. The number of players in the fallback hoppers has dropped, however there are still people experiencing problems accessing the main servers. We’ll continue to investigate and push out fixes until these issues are resolved.”
The ride may be bumpy for now, but the team is actively working on a fix. During this downtime, why don’t you run through the campaign on Legendary? It shouldn’t take you that long, and when it’s over the servers should be fixed!