[SOLVED][Drupal Errors][Drupal 6] Page Not Found error on all pages of a Drupal 6 website

You will get this error in your Drupal site if your menu router data got corrupted or deleted in any way and this will occur during the process of enabling a module. Follow the steps below to fix the issue.

• You should start by disabling all non core modules to find out which module is the suspect module.
• Since the site is not accessible you have to do it via SQL.
• You should set the status field of the suspect contributed module(s) to 0 in the system table.
• After you have done that you need to rebuild your menu system by running update.php.
• If running update.php does not work then find the menu_execute_active_handler() function inside /includes/menu.inc and force it to call menu_rebuild() as follows.
• Change the code from
  

  if (variable_get('menu_rebuild_needed', FALSE)) {
      menu_rebuild();
    }

  to this

  //if (variable_get('menu_rebuild_needed', FALSE)) {
      menu_rebuild();
    //}

• After doing this try to access the site and the menu will automatically be rebuild
• Remember to remove the changes you made to the code once everything has started working again.

Hope that helps.