I'm no expert but I would look over your Google Webmaster Tools stats to see if there is any relevant information. You should be able to see a timeline of the number of pages listed in your sitemap vs. the number of pages indexed in Google.
The way Google operates these days, they could have removed your page for duplicate content, weak content, copyright concerns, outside spam links, non family friendly content, deceptive ad placement, or for no observable reason at all.
There is also the "site made for search engines" penalty that drives me crazy as there are suggestions from Google regarding how to make you site more "search engine friendly". If the bots don't like your page, there isn't much you can do after the fact.
My advice, if you cannot figure out the issue would be to move on. Delete the page and create it again in a different form.