Sometimes when users update templates / plugins or even WordPress itself, the site can get locked in Maintenance mode.
This means that any attempt to visit either the site or the back-end will be met with a blank page with this message:

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

To ‘undo’ this situation, log in to cPanel, then open File Manager. In the site root, look for a file named .maintenance and delete it.
The site will now load again.