by Emma Richardson.
When a site is in maintenance mode (since about 2.5 or 2.6) I have noticed that you have to manually type in the url for the login page. I think it is by design to stop people trying to log in while in maintenance mode. Now you are logged in, you should disable maintenance mode through the web interface in admin settings and then comment out the line in config.php so that you can enable without having to mess with your config file.