The outage, which began Monday evening and affected a "significant" number of MSN Messenger users, was fixed by about 5 p.m. PST on Tuesday, an MSN spokeswoman confirmed.
MSN blamed an isolated data-center problem for the outage, and the spokeswoman stressed that the so-called W32/Bropia worm hitting MSN Messenger was not responsible for the access problems.
"Weve identified the cause and are working to take the appropriate steps to remedy the situation as rapidly as possible," the spokeswoman said earlier.
MSN officials had no estimate on the total number of users affected by the outage. MSN Messenger has about 145 million users worldwide.
MSN Messenger has battled outages before. In March, it battled two separate outages, each of which lasted for at least three hours and coincided with access problems for other MSN services.
MSN is not alone among major IM providers in facing service downtime. Most recently, in December, America Online Inc. accidentally disabled the accounts of as many as 10,000 active users, leaving those users without access to AOL Instant Messenger for almost a week.