Have you recently upgraded or updated your Drupal site? Updated it enough to where the URLs have changed? This is bad for people finding your site via search engines. The search engines are still linked to your old site. What happens when someone clicks on the link? They get a Page Not Found on your new site. This is not good.
Search 404 Module is the answer. It takes what you were looking for and searches your new Drupal site and returns the results it found instead of the Page Not Found. In this video I show you how this works and what different options you have.