[Plugin:WordPress Automatic upgrade] Upgrading to new version.

From the readme.txt file for that plugin:

= My site is in maintenance mode and the plugin did not complete. What do I do now? =

Only under some rare condition will this happen. To get your site back online you need to do these things.

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.

Link to see problem, please.