Not sure how I can explain this...
MSN has quite a high 'setting' to establish that you're online, if you run lots of things and slow your connection down too much it'll think you're offline and log you out.
Failing that check the 'Help' option and see the status of your local MSN server. Here in the UK it played up for about 2-3 days while they carried out maintainance. We got thrown off, couldn't log in, couldn't recieve or send messages etc.
Can always try e-mailing MSN but good luck on a reply!