The first-person-shooter game is the sixth instalment in the series.
An update to blockbuster game Call of Duty: Modern Warfare 2, which disconnected gamers from Microsoft's Xbox live service, has been fixed.
The problem was caused by the release of the Stimulus Package, an update for the game which contained new levels.
Gamers who purchased the update found they were unable to log on to the online service to play the game.
Microsoft has said that it has now resolved the issue, although admitted there were still other problems.
"We have deployed a title update via Xbox LIVE which has resolved the issue, and the content is now functioning properly," a spokesperson said.
"We are also aware of some other service issues that some are experiencing related to billing and match-making that we are working to resolve quickly."
BBC News was originally tipped off to the problem by reader Stephen Reynolds.
"Modern warfare is now not playable by me, any of my friends in the UK, and I'm sure thousands others," he said.
The game was the best-selling videogame of 2009 in the UK, according to industry body Elspa. It was one of only a handful of games to make more than $1bn (£625m) worldwide.
The Stimulus Package adds five new maps to the multiplayer version of the game.