During recent google update one of site came on no 3 with the keyword search engine optimizer and i become very happy. But after the update was over i cannt find it even in top 1000....really great google...
It can happen to everyone ........happened to me just don't worry about it and keep adding content and find other methods to get visitors for now and improving your site. I'd like to say you can rely on Google but you never know with those guys.