On 404 Not Found error pages, Drupal will skip rendering of all the navigation links of your website for performance reasons*. Some of these include:
- The "Primary links" block and any other menu links block.
- The Primary links and Secondary links of your theme.
* Unless you have configured a "Default 404 (not found) page" on admin/config/system/site-information.
But many websites find navigation invaluable. Especially on 404 pages, when they want to show lost users how to get to real pages.
So this module simply revives those features on 404 pages. Simply install and enable the module. No configuration needed.
For a Drupal 5 or 6 version of this module, see the 404 Blocks module.