If you have been given a manual penalty, it will be seen in Google Webmaster Tools under Manual Actions. If you have not suffered a manual penalty, then your site has just been de-ranked because of the regular algorithm. Use GWT to ensure your site is being crawled correctly, look at your highest traffic pages and find out which ones dropped in page position. If you can find specific pages that got dropped more than others, you can look for problems associated with those pages, especially back link profile. If you got a manual penalty, you will need to disavow links. If it's just the regular algorithm, you need to work on building high trust backlinks to recover.