It doesn't matter that you ranked for it before. According to the new algorithm your site is no longer the best to show. Period.
I understand what you are trying to say, but this is simply not the case here. For the main keyword that I am describing we do not even feature in the first 300 results, and we are talking a fairly low competition keyword. Plus if you add another word on to the keyword in question to make it a two word search phrase we rank number 1, even above Wikipedia.
My website has a great link profile and relevant content all relating to this keyword so it is clear to see that it has been penalised by the Penguin algorithm. Once the algorithm has been refreshed and my website (hopefully) returns to the rankings for that particular keyword I will report back here to prove my point.