I'm sure it's banned. Were you doing too much reciprocal linking or any over-optimization recently? Did you email them?
I don't think there is any specific way on how you can get indexed in MSN. Just keep optimizing your site. Sometimes it really takes time.
It's likely due to reciprocal linking. Any type of links they feel are non-relevant can trigger a ban and they don't seem to have any support that will even respond to you. It doesn't affect their bot which will continue to visit the site - they just won't include it in their index.
I just checked some of my sites...and MSN is not indexing any of mine either....although they used to. I used to get tons of traffic from them, and was high up on the list...now I still get *some* traffic, but I'm not indexed...this is why I hate live.