Recently we have taken on projects where the client's site has been subject to penalties incurred from Google's recent Panda and Penguin algorithm updates, which targeted unnatural or indiscriminate link building activity, predominantly undertaken by SEO agencies as a quick and dirty alternative to a recommended high quality content marketing and outreach strategy.
Here is a great, detailed case study from expert SEO consultant Gary Viray on steps taken to successfully recovering a client in a similar situation from the same drop in Google search traffic. It is a really practical explanation of steps to recovery with corresponding examples from the case study
A True Story: From A Triple Whammy Google Penalty To $60K/month Revenue