A page or a full website? And have Google confirmed that it’s banned?
Sometimes pages drop out of the index – some come back in a day or so, some don’t for a long time.
Have you checked all the obvious stuff like broken links, hidden text, duplicate content (via copyscape)?
I had a problem where I’d changed the foreground and background colours on a text box, but left a few trailing spaces at the bottom. All affected pages were dropped from the index as they showed up as potential spam/hidden links.
You can be 100% white-hat and still inadvertently fall foul of Google!