Microsoft''s MSN Messenger service went down Monday, leaving most of the instant messaging software's 75 million worldwide users without access to the service for more than five hours.

According to a Microsoft spokesman, the service went down at approximately 6 a.m. PST, and the root cause of the outage is still unknown.

The outage affected all of Microsoft's .Net Messenger Service users worldwide, including Windows Messenger and MSN Messenger subscribers, according to a statement from Larry Grothaus, lead product manager for MSN. .Net Messenger Service is the back-end service that powers both the Windows Messenger and the MSN Messenger clients. The problems occurred somewhere in the .Net Messenger Service, Grothaus said. MSN Hotmail e-mail service and other MSN services were not affected, he added.

Although service was restored for some users by about 11 a.m. PST, a number of users were still unable to log on to the messaging software into the afternoon. Microsoft is not seeing any more difficulties with the service right now, but some users may still be shut out as the service scales back up, according to Grothaus.

"It looks from what I've seen that [the service] is scaling back up. Some users may be having a little trouble getting on, but it just because it is just an issue of scaling the service back up and getting everybody back on the authentication servers," Grothaus said.

Customers can obtain .Net Messenger Service status information here.

Last week, Microsoft's Passport service suffered an outage.