Fix : WordPress is Stuck in Maintenance Mode after Automatic Upgrade

Is your WordPress site stuck in maintenance mode and displaying the following message?

Briefly unavailable for scheduled maintenance. Check back in a minute.

Deleting the file called “.maintenance” in the root of the site’s public folder should fix the maintenance mode issue. In most cases the file upgrade would be done successfully and progress stuck at the database upgrade stage. After deleting this file, you might have to hit Refresh on your browser to release the maintenance mode. Then, you will be prompted for a database upgrade, if it is required.

Source: http://wordpress.org/support/topic/30-stuck-in-maintenance-mode-after-failed-plugin-upgrade