The Google panda is updating itself from time to time to make the advance and relevant stuff available to its browsers. It is changing its policy since 2 months to get up to date. What you require to do is to do the SEO according to current changes and guidelines of Panda and Penguin. This will get your site back to the relevant position and actually on the top of its position.
Many sites have been hit hard by the new google updates. Many of us have to start from scratch when it comes to seo. Now it doesn't mater how many links you have to your site but the quality of links to your site. Just remember it is better to have links in content of a web page like a article keyword rather then have a bunch of single links on peoples homepage to your site.
In the recent Google update, aside from SERP, my pagerank has also been affected. Most of my blogs' pagerank decreased by one point. I guess it's really difficult to predict Google's algorithm because it varies every update. it's like starting from scratch again.