After getting hit with some serious forum spam I put my one Drupal site into maintenance and forgot about it for a couple of weeks. When I came back to update it to the newest Drupal version I found that I had been logged out and no longer got a menu with it in maintenance. I knew that in WordPress I could add /wp-admin/ to the URL to get to a login screen but I didn’t know the equivalent for Drupal.
To log into your “under maintenance” Drupal site simply append /?q=user to your address
From there you can log in and take your site out of maintenance mode, or apply whatever tweaks you need to perform.