If you're in the same situation as I am, it's because there's a problem with billing. For some reason, at some point in the past five hours or so, a ton of people were locked out of their accounts and MoP was deleted from their account. The issue apparently is that Blizzard hasn't taken their money from those people's banks accounts, and won't be able to play until they do.
You can try calling the Customer Service line, but they'll just tell you the queue is full and you're not even allowed to join the waiting line. I guess at this point, unlucky people like myself and the few I've seen in Orgrimmar will just have to wait it out or wait til Blizzard makes enough time for us.
Here's the link to the blue post
Edit: It seems like it's been fixed now. It seems to be happening to people who use a card different from what their normal billing card is. I re-entered the information from the same card number (the first transaction never went through) as per Blizzard's instructions and MoP is up again. Guess I'll know in a few hours if I lose MoP again for the same reason.
Anyone else having similar issues?