There was a Panda data refresh on 19th April, 2012. You also need to check if your website was penalised by the very latest ‘Over optimization’ penalty or the even latest ‘Penguin update’.http://searchengineland.com/the-peng...al-name-119623
Here’s Matt Cutts helping you with other possible reasons:
Techniques to follow now?
Avoid indulgence in keyword stuffing, link spamming & linking the same anchor text to multiple sources. Include unique, informative content in your website and make the most of social media interactions.