Search Engine Watch Forums has a discussion on sites banned by MSN. One of the guys had no rankings at all and search for his domain didn't even show it. The MSN support team asked him to check the robots.txt file.
There are many others who are facing the same problem and got in touch with the support team. Here is a response which just about summarizes it:
Same happened to me too back in Mid-May – have sent several emails to support and received about 3 back – all clever responses that have just not really read my emails..
I keep getting an MSN dude called Joan!! my site has been in msn for 5-6 years and had top 10 traffic on car insurance for the last 2 or 3 years then bang! it all went in may this year.
However, in an interesting development MSN has sent out, what seems like the first public mail for banning a site for link exchanges. The mail reads:
Your site is acquiring links through posting to or exchanging links with sites unrelated to your site content. Techniques which attempt to acquire unrelated spam links in order to increase ranking are considered spam and your site has been excluded from our index as results. Please contact us once you've removed these links and we will reevaluate.
Join the discussion here.