Do you mean the site or the messenger service? MSN Messenger was down for the best part of yesterday evening (6-10pm GMT roughly - I couldn't sign in between those times and nor could anyone else by the looks of it) but it seems to be back up now.
Im talking about the MSN Messenger.. Its weird, still looks bad for me... guess I'll have to wait a little bit more than.
They have an update today for a new version, that may be why you are having some problems. I use trillion and it was down most of the day also for MSN. but seems to be fine today.