Google recently released an update to their SEO algorithm that effected a lot of sites. It is called Google Panda update. This time Google seems to have not got it 100% right as a lot of very good, very valid sites were heavily penalised. For a site that was penalised it is a long hard work to get it back often having to change content. If you need more info you can contact us as we have a team of SEO experts who can help. You would basically see a heavy drop in traffic around April this year starting form USA and now recently over all countries.