A company representative declined to elaborate on the nature of the problem, or the steps Microsoft took in fixing it. In an earlier statement, the representative said the outage was caused by an "isolated issue that we've located in the data center."More info at Cnet
The problem had affected a "significant" number of MSN Messenger users intermittently since Monday afternoon, the representative said.
MSN Messenger outage caused by data center issue

Yesterday millions of people were unable to use Microsoft's MSN Messenger service due to some data center issues.