My keyword ranking suddenly went down in google.

[quadradesign2]

This might be because of Penguin Update from Google.

[engineeringdoc]

Sometimes google changes algorithm and according to that website will changes ranking.its very hard to maintain good ranking in the search engine.

[gapinfotech]

it's not a fault of penguin update my be fault in your side like some link remove or if your change your seo activate so ranking will be down

[Vicken]

I will not say penguin, If your site rank goes two or three pages down it may come back. Normal fluctuation in ranking is may be due to decrease in backlinks.

[solidtrustpay]

Google penguin and panda effect updating to affected your keyword. Google thinking you are spammer so suddenly down your keyword ranking.

[nanv]

I think fluctuating is normal in SEO but make sure you are following only white hat seo and be aware from the Google Panda and Penguine updates.