On December 31, they wrote,
“Today, we experienced a bug that caused Season 3 to unexpectedly end for Arenas and Rated Battlegrounds.
We’re investigating and working on fixes for this situation. We do not yet know how long this will take.”
They posted an update today in which they stated that they’re currently working on a hotfix for the issue to resume Season 3, but this hotfix isn’t ready yet.
“We’re working on a hotfix for the game that will resume Season 3, and at this time, we expect that we will be able to retroactively retain rating changes and tier updates. We do not expect that we will be able to retroactively retain the season games played count or win counts that increased during this brief offseason.”
Once the hotfix is ready, Blizzard has said they’ll post an update, which will necessitate a restart of all realms to deploy the fix.