Affiliate site has been hit by Panda

[batman]

You were ranked in the first page that means you have pretty much content in your site already. But now after the Panda update it has fallen that means Google does not think that your content is unique enough.
Try to analyze your competitors content, their keywords. You can also try the Skyscraper technique to build unique content. Build outside the box content. As soon as Google realizes that you have much unique content, your previous rank will be regained.