Hearthstone's new Curse of Naxxramas singleplayer campaign can rest assured that developer Blizzard is looking into the problem.
Following yesterday's debut of the Plague Wing area, reports came rolling in of players unable to purchase the additional content or being double-charged by the game's in-app purchase system. "We are currently investigating the following issues affecting the new Hearthstone wing, the Plague Quarter," Blizzard wrote in a statement to Eurogamer. "Some players are unable to access the new wing after purchase. Some players have been charged twice in order to access the new wing. We apologize for the inconvenience and thank you for your patience while we work to resolve them."
Since issuing that statement, Blizzard has created a notice in the Hearthstone forums that offers guidelines for those affected. If your purchase failed, securing a refund should be a simple process, though Blizzard continues to look into those cases where payment was successfully made but players were not granted access to the game's new features.