Never heard of Google banning a single page. I think it just de-indexed for sometime as they might be updating something. Don't worry it should appear in SERPS soon.
I've noticed my blog posts on Google fluctuate quite a bit - sometimes I am #1, sometimes I am on page 3 and sometimes my post is not there at all even though it was number 1 just a few days ago. Google is all some wierd science that I will not even attempt to try and understand. There are a lot of ways to get traffic outside of Google and if you stress yourself out about your Google placements, you will make yourself go nuts ... like I did in 2006
A couple of my sites have been bouncing around in Google for a few days now... Just keep working it will make it back up there.
It wasn't banned, or even "de-indexed" More than likely not all the data centers had found it yet. When you saw it ranked, that certain data center had found it. When you checked later, it was a different data center that had not.
Google has hundreds of datacenters and the index is not 100% synchronized across them. So, the Google search results seem to dance. Here is an interesting tool that you can use to check your results across the different Google datacenters http://www.mcdar.net/dance/index.php
Google love new content and gives a New Content Boost to new sites or new pages. This boost help them rank well in search results. But this boost is temporary. You will see new pages search results rank change often
Do you consider your particular page to be sandboxed? As far as i know, google doesn't penalize one single page. They either love you by indexing you or they completely ban you or place you in their sandbox.
Your server probably returned a 401 when Google went to take a look at the page. They will try to cache again in a few days - this is nothing to worry about.
A single page may be banned. This happened to me because an error: two successive redirects to this page. Other reasons are possible.