Google's search guru Matt Cutts has been hosting a rather popular series of YouTube videos designed to help website owners diagnose problems, find out about more complex issues or simply ask questions that they haven't been able to answer otherwise.
More recently,
Cutts started doing somewhat longer videos where appropriate and Google has now highlighted one such video which aims to help webmasters that have found their website ranking dropping mysteriously.
"When you notice a drastic drop in your rankings and traffic from Google, what process would you take for diagnosing the issue?," was the exact question.
The video clocks in at about five minutes and there are plenty of useful tips in there, from checking whether Google and other search engines are indexing your site at all or just partially, to requesting a reconsideration for your site.