Sounds like there is a connectivity issue which is preventing MSN from functioning correctly. My best guess would be to try and figure out what is causing MSN to stop.
With that in mind, you may want to check the following:
1) Firewall - Try temporarily disabling this. If MSN works then you know that its the firewall thats blocking it and will need to allow it rights to communicate. Don't forget to re-enable it afterwards.
2) Spyware - If you have received a file via MSN and found that afterwards you haven't been able to sign in, or been on any of those scabby IM sites that give you daft smilies, then spyware may be the problem. Google Spybot Search and Destroy, install it and see what it finds.
3) Use the diagnostic tool in MSN Messenger under Tools. This will do all wonder of things to try and make a connection. See if this does anything for you.
Urban Kaos, January 2008