Google doesn't "ban" sites from its index without very significant reasons. Sites are more likely to experience some sort of de-ranking while questionable activity is investigated. The type of activity which would be considered potentially ban-worthy would include cloaking - wherein pages that are presented to the search engine differ from those presented to site visitors.
theres a million and one different reason paid links, malware etc.. nice post here on it all davidnaylor.co.uk/closing-down-the-blog.html (not linking for you dave
Yeah as already mentioned there are loads of reasons your site can be removed from googles index, generally its quite easy fix though, just stop doing what they dont like and you will soon be reincluded again.
Check out what you have done recently that may have affected your rankings, it's an annoying problem to face if you have done nothing wrong. Keep adding content and building relevant quality one way links and you will be back up in a short while. We had a clients site banned, we continued as normal and within two weeks it was back up to the top spots on Google, even though we didn't do anything wrong, so sometimes it's just one of those things. Carry on as normal and don't panic. Regards, EIMG
hello, read google webmaster guidelines..you will find all the specified reasons behind banning any website.