Sometimes, upgrades fail at some point for some reason. That will leave your site in maintenance mode and unusable. That's the "Briefly Unavailable for Scheduled Maintenance" that we see while the upgrade is on-going or if it fails. So how do we get out of the maintenance mode?
The fix is as simple as deleting the file named .maintenance in the root folder of the site. You can delete the file via your favorite ftp or ssh client.
During upgrade, WordPress creates a .maintenance file in the root folder. This file is removed after the upgrade is finished.