This has been notified - do not panic, it is not just you. This problem has been happening for a few days to many people. Msn is aware (see their homepage) and are fixing the problem. Should be fixed within the week.
Use webmessenger until we figure out how to solve it.
http://webmessenger.msn.com or http://iloveim.com
Check also MSN Service status: http://status.messenger.msn.com/Status.aspx
Update: It seems the service is returning to normal. I would suggest signing out and back in again if you are still receiving the error. Apparently engineers investigated the problem and found a fix.
If none of these methods listed above won't work, we recommend you to run registry fix tool.