The plugin is programmed to filter out /wp-admin and /wp-login.php from the redirect hook. If you are trying to login in your site from /wp-admin and are faced with the Under Construction page, please go to yoursite.com/wp-login.php.

If you have a security plugin activated that changes the login path, it should still account for /wp-login.php.

If all else fails and you are still unable to get in, you can FTP into your site directory and delete the security plugin, or delete WP Construction Mode. Once you gain access to your site again, configure the security plugin in a way to account for /wp-login login attempts, then re-activate WP Construction Mode.