A sad fact of the SEO world is that sometimes, websites get manual action penalties from Google as they have been too aggressive in their optimisations and have triggered a Google penalty from the web spam team.
Whether this is link based due to the penguin algorithm, content based due to the Panda algorithm or if you have not made your site EAT friendly (expertise, authority and trust) and have fallen fowl of Google's search quality guidelines, you need to put a plan together to remedy the issues as fast as possible.
At Type A, we do a penalty recovery at least once a quarter (sometimes more often if Google is feeling a little trigger happy with their updates). And have done it across a range of YMYL sites (Your Money or Your Life) and have confidently return peoples rankings in anywhere from 2 weeks to 6 months. There are usually no quick fixes to getting out of a penalty (especially algorithmic penalties) but we have had a 100% success rate in getting our clients traffic back. However, be aware that some take longer than others depending on the severity, industry and type of penalty you have received.