Some of the players suggest the issue started on the same day as the release of the Spartan Ops missions. According to an official post on the Halo Waypoint forums, the problem seems to be with people being stuck on the fall back server and therefore being unable to connect to the main server instead.
The Halo developers want you to help by providing information on the issue along with your gamertag and region:
“There are a few hundred players who have been unable to connect to our servers since yesterday. We’re investigating now and hope to have an update soon. If you are experiencing this issue, we’d appreciate knowing your gamertag and what region you’re in, as this will help us with our investigation. Thank you in advance for any additional insight you can provide.”
You can read the forum thread here.