The second wing of the Curse of Naxxramas expansion for Hearthstone: Heroes of Warcraft was opened yesterday, or at least that was the general idea.
Instead, players met all manner of difficulty, where they were unable to purchase the new wing, or were charged for it (sometimes twice) but still couldn't play it. This resulted in some understandable annoyance.
Blizzard has tried to calm down the rumbling mob, and promises to sort out the situation as soon as possible. In the meanwhile, players might want to hold back on buying the Plague Quarter for now.
You can follow how the situation progresses on the Hearthstone forums.