Blizzard has issued an official statement regarding the Arena being disabled. The Arena has been disabled due to issues after today's game update.
UPDATE: 10:36pm PDT on June 3
We’ve resolved the issues with arena mode and expect to make the mode available again shortly after this post goes live.
The 14.4 update for mobile will now begin at or around 9am PDT tomorrow morning, June 4. The update will take some time to complete, so please allow an hour or so to see the update reflected on your device. We will provide updates during this time as well so you know when to expect to see the game live.
We’ve identified an issue with Arena caused by the most recent Hearthstone update, which requires us to temporarily disable the game mode. During this time, Arena will not be available for play. Please check back on this thread for progress updates on the issue.