Hi dear, This may be due to Google update. don't worry about this, just keep on doing quality work for your site. Thanks.
This may be because of Google panda update 21. Cay you tell me your keyword position before getting down and presently.
If you havent done some black SEO then you do not have to worry. Sometimes it happens after a Google update but you will regain it soon. Keep doing the right thing.
There always ups and down. Some people's keyword goes very down and some effected little bit. Just do your work in right way, don't do over optimization. Submit articles in high pr article sites.
It may be because of thin content by Google panda update on November 5 2012. You should check content and make sure for thin content.