If your website’s position in Google for a particular search query has suddenly dropped from the first page to, say, the fifth page of Google search results, chances are that your site has been manually penalized – maybe you did something that is not in accordance with Google’s webmaster guidelines.
Well I hope none of us would ever need to use this but in case your site gets penalized in future, you can always ask Google through a reconsideration request and they may offer you an hint for debugging in the right direction. The Search Quality Team writes on the Webmaster blog:
Now, if your site is affected by a manual spam action, we may let you know if we were able to revoke that manual action based on your reconsideration request. Or, we could tell you if your site is still in violation of our guidelines. If your site is not actually affected by any manual action, we may let you know that as well
