If Drupal 6 runs into a fatal problem while calling hook_menu(), your site may get into a state where all page views (the front page, the administration page, the modules page, everything) will give a 404/"Page not found"/"The requested page could not be found" error.

You can revive your site by fixing the error that caused the menu rebuild failure and then loading /update.php and completing the upgrade process (even if there are no updates available for any of the modules on your site).