Search engines are constantly refining the quality of their search results to improve user experience. Over the years major search engines such as Google or Yandex have released various changes to their search algorithms, in order to crack down on unscrupulous SEO practices. These involve updates such as Google Penguin or Yandex Minusinsk which penalise websites having unnatural inbound links, usually involving buying and selling links, to try to artificially inflate websites' trust factors and page position purely to get better rankings. Both Yandex and Google issue algorithmic penalties. Google is also known for imposing manual penalties.
It can sometimes take a while to recover from a penalty, as the search engine won't inform you why your site has been penalised. You'll need to consult an expert who knows what to look out for and identify potential causes of the penalty along with recommendations on how to reverse this and restore rankings of your website.