Errorcode: 0x8100306 or 81000306 (with MSN Messenger 7)
"We were unable to sign you in to .NET Messenger Service, possibly because of a problem with the service or with your Internet connection. Please make sure that you are connected to the Internet."
If you did not have this problem before, this usally means the MSN server is not reachable. It may be down for a while, usually if you just wait a while you can connect to MSN Messenger again.
It may also indicate you have a proxy server or firewall on your network that does not allow MSN Messenger to connect. This is usually the case in companies or schools. Ask the network administrator how you can use MSN Messenger and if they will allow it. You can also use a web based messenger in this case.