Earlier, it was almost impossible to diagnose the reason for a Google penalty and website owners could only guess whether their site was affected for violating guidelines or because of other factors beyond their control – maybe another site had better content and that could have pushed your site down in Google without you doing anything “black hat.”

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:

If your site has been manually penalized in Google for violating guidelines, you need to file a reconsideration request and wait until they revoke the penalty. If the site is not ranking due to a bug in Google algorithms, or because of access issues (like your server is down), they’ll let you know that as well.

Here’s one of the older videos from Google that talks in detail about reconsideration requests – the Webmaster tools UI though may have changed in these years.

If your site has stopped showing in Google's search results, there are some steps you can take to help reintroduce it to Google. From checking the site's robots.txt file to submitting a reconsideration request, this video tells the story of one webmaster investigating his site's disappearance from Google.