The game is currently down to run the script that will unlock all decks currently set to the "Beginner's Arena" and remove all illegal decks from the Limited and Standard arenas.
All decks by all players will be removed from all arenas as part of this fix. When you get back into the game, you will need to reselect your Arena decks before battling.
We expect this downtime to be completed by 11:30 am EST.
UPDATE: The script was running too slowly in the live environment, so we have decided to stop it, make some changes and run it again on another day. The game now open.
In the meantime, if you want to unlock the deck slot that is set to Beginner's Arena, you can do so by doing the following:
1) Go to the Training Grounds
2) Select any Hero Challenge, like the Amorya Challenge in the first area.
3) Click on the deck to edit it.
4) You will receive a message saying that this will remove your deck from the arena. Click "YES"
5) Your deck has now been removed from the Beginner's Arena. You can edit if you like or you can just close back out of the deck editor.