At the end of 2012 we moved a web site from Joomla to Drupal and from http to https. During the move, for a couple of weeks we used a 302 redirect rather than 301 and we almost disappeared from Google.
We fixed that and also discovered a number of issues with Drupal plugins that created duplicate pages which we have addressed and almost immediately started recovering in the rankings and search results.
However, the site's page rank went down from 3 to 0 and at 0 remains today, suggesting that there may be something else going on.
Realistically we need someone with experience with Drupal SEO and possibly google penalties to audit the site's structure and make sure we use all the SEO best practices for Drupal 7 sites.