A hacker group is taking responsibility for taking down Xbox Live's online service last night.
By Tuesday morning, Microsoft said all its Xbox Live services were "up and running."
Xbox support worked to resolve the issue overnight, the company tweeted from its verified support account in response to several user complaints about lack of access to the online system.
"We're currently working to resolve this. Thanks for your patience," one of Xbox's support team members tweeted just before midnight Monday.
The company said it was a "busy night."
Lizard Squad, a hacker group, claimed it was behind the attack.
"Xbox Live #offline," the group tweeted at 5:37 p.m. Monday, later adding that the shutdown is "a small dose of what's to come on Christmas."
Lizard Squad calls itself the "ddos kings" in its brief Twitter bio. Ddos refers to distributed denial of service attacks —the group's primary ammunition.
It has previously claimed responsibility for shutting down other gaming services, including Sony's PlayStation network in August.
In a separate incident, the Lizard Squad claimed through its Twitter account that explosives might be on an American Airlines flight carrying Sony Online Entertainment President John Smedley. His plane was diverted.