This error occurs when you try to update something in WordPress site and the update fails for some reason and then your site goes down and following message is displayed
Don’t worry. The process to fix this is pretty straight-forward. Go to your blog installation folder and look for .maintenance file. This file is generated by WordPress when the update occurs and then removed after the update finishes. In case update fails and this file could not be removed, then the above error message is displayed. Delete this file and you will be back up.