You might have found yourself in a situation where WordPress updates to a newer version and makes the maintenance mode active, however for some reasons it didn't deactivate the maintenance mode and you get the "Briefly unavailable for scheduled maintenance. Please check back in a minute." message. Usually this message shows only for a few seconds while WordPress installs the new files on your server, but sometimes it doesn't finish the process and the message is still shown on your page.
Let's look a little deeper, when WordPress starts an automatic update it creates .maintenance file on your server which shows the message that the website is under a maintenance. However if it is interrupted during the process and the file doesn't get deleted the message stays shown on the website until you manually delete the file.
To delete the .maintenance file you can do it either using the FTP client or using the cPanel. I'll show you both methods in this article.
Open your FTP client such as FileZilla, Total Commander, etc. and connect to your server.
Once connected go to the root folder of your WordPress installation and look for the .maintenance file and delete it.
Log in to your cPanel and go to File Manager. Open your WordPress root folder and find the .maintenance file and delete it from the server.
Once you have deleted the .maintenance file your website should work normally and if the WordPress update didn't install correctly it will re-do it now.