I heard from many people that because of Google Panda algorithm many people's sites have been affected in keywords in Google.Maybe your also be same reason.
In the month of February 2011, Google updated its algorithm for quality search which is called Panda Update. It affects website having copied or low quality content. May be your website get a penalty according to this update. We are able to tell you exact reason after analysing your website. You can PM me your websites URL
Update your website content. This time make it more unique, increase the amount of content, maintain a proper keyword density, get backlinks and social media links as many as possible and make sure that they are relevant and of good quality.
If it would traffic related problem then you could be notified by Google. If you not notified by Google yet then it can be reasoned by Google panda.
Panda algorithm is Google's battle against poor quality websites. Few days back, Google rolled out this algorithm after which websites with high quality content jumped up in the search results where as other websites have been pushed down. I think, your site is among those who are strongly hit by this update.
Try to diversify your content. I have found if you include images, youtube videos, etc in your content then it helps. Also if you are just using generated content and not your own then this just shows that Panda did what it was designed to do. I am not speaking against generated content, but I am saying the idea for Google was to push these sites down.
Google Panda updates have done a strong ranking effect. You site may also effected from Panda. Just wait and see for couple of weeks to see whether things improve. If not start build link popularity and focus on adding more contents.
A quality content with adequate density of keywords which clearly specifies what your website is about should be your main concern after being affected by panda algorithm.