If you were hoping to play World of Warcraft, Hearthstone, or any other Blizzard title last night, you probably had issues connecting. In fact, all of Blizzard's US servers were down for an extended period last night. There was no scheduled maintenance causing the downtime, instead the culprits were a group of Lizards.
The hacker group known as Lizard Squad decided that they needed to target Blizzard's US servers last night, and they appear to have made quick work of them. All Battle.net games were offline for an indeterminate amount of time last night and this morning. The hacker group took credit for the attack, and were sure to tweet about it just before they began their assault.
Blizzard responded to the attack by confirming that they were in fact hit with a "potential DDoS" at one of their datacenters. They then stated that they had taken actions to bring their services back up, and would continue working to mitigate the impact. A bit later they confirmed that a second attack had happened, but their "supercool internet-shield" stopped it.
Many people are wondering why Lizard Squad picked this time to focus an attack on Blizzard's service. There are a few theories floating around, though the group hasn't confirmed their motive. Personally, I think that it might have something to do with the legal action against private vanilla WoW server, Nostalrius. Perhaps some members of the Squad enjoyed playing on the server, and decided to see how Blizzard enjoyed having their servers shut down.
Regardless of the motive, Blizzard appears to be unharmed after the attack, and a large number of players were inconvenienced for a few hours. There are also reports of the team hacking one or more Blizzard staff email accounts, though Blizzard hasn't confirmed these.