This is rare because most of the time the site will be removed from the search results by Google Panda. Occassionally however you may have fooled the algorithm with activities such as link building but ultimately a manual check has caught up with you.
Alternatively you may have been caught out by not realising you had large numbers of pages which were being automatically generated and all of which contained little or no substance. Overuse of the tag system in Wordpress is a common cause of this. You can check for unexpected pages in Google Analytics under Behaviour -> Site Content -> All Pages.
To resolve this issue the steps are the same as Google Panda Recovery with the exception that you will also need to submit a reconsideration request to Google once your new content is in place.
As such you should keep a careful, dated diary of the content you created in order to put forward a convincing reconsideration request.
I'm Tim Hill, a Search Engine Optimization and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.