Almost every day Google slightly changes the way in which it ranks websites for particular search queries. Many of these changes to its algorithms are barely noticeable but sometimes a major change occurs that is accompanied by actionable official advice from Google (a good example is when Google recommended that all websites be secured using an SSL certificate). Several times per year Google makes more significant broad changes, referred to as "core updates", that have more wide-ranging effects. Google has now released official advice for any websites that experience significant changes in rankings after core updates.
Sometimes websites that engage in deliberately misleading Search Engine Optimization (SEO) strategies will be deliberately targeted by updates to Google's algorithms , such as with the "Doorway" Update in April 2015, but pages that perform less well after a core update don't have anything specific wrong with them and have not been subjected to a targeted action. Instead, Google stresses that the changes are "about improving how our systems assess content overall" and uses the example of the fact that a list of top 100 movies will change from year to year due to the existence of new movies and a re-assessment of older movies.
If you want your website's ranking to recover to where it was before a core update, it's content will need to improve relative to its competitors. Google has issued the following advice for website owners who have been impacted by a core update:
Any improvements you have made to your content will generally not benefit your rankings until at least the next broad core update, which take place every few months. However, you could be lucky and experience an improvement due to one of the more frequent, but much less wide-ranging, incremental updates. It is also worth keeping in mind that any changes you may make in relation to Google's recommendations will not necessarily benefit your website's rankings if other websites are more deserving, and that even if you make no changes your website might recover due to further core updates in future.