According to me you have to upgrade plugin.This plugin only upgrades the files that are essential which includes the file in wp-includes, wp-admin and the root directory.
1. Login to your site using FTP.
2. Delete the index.php file.
3. Rename the index.php.wpau.bak file to index.php.
Performing these steps should remove the maintenance mode message.
1. Login to your site using FTP.
2. Delete the index.php file.
3. Rename the index.php.wpau.bak file to index.php.
Performing these steps should remove the maintenance mode message.