Could you please tell us when exactly this occurred? Last year there was some mayor Google algorithm updates like Panda and Penguin this year 2015 so far I am not aware of any!
Check your backlinks I am pretty sure it could be cause you lost some of them or you got too many spammy ones! Also honestly have you been doing black hat SEO?
Check you webmaster tools account for manual actions!
Please share your URL and tell us what keywords you are targeting!
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