The servers have been down but are temporarily up
While they may be operational, there is going to be maintenance performed on them starting tomorrow
You can keep up to date with that in this thread: https://www.halowaypoint.com/en-us/forums/36cecc379ca14abd918029d359551927/topics/reach-halo-3-odst-maintenance-5-29—5-30/408b60be-3604-4769-bd4e-bccccba64e38/posts