When doing updates in WordPress, upgrading the WordPress version or the Plug-Ins causes the site to enter into Maintenance Mode. While in Maintenance Mode, a message appears that says “Briefly unavailable for scheduled maintenance. Check back in a minute.” rather than the actual site. Sometimes, especially if you’re using the automatic updating functions, an update might fail and the site may be stuck in Maintenance Mode.
WordPress looks at the root level of a directory for some hidden files that can tell a site to operate in a different manner. If there’s a file called “.maintenance” then the site will display the message above. When an update of a Plug-in fails, the .maintenance file is never deleted and the site is stuck in Maintenance Mode. To correct the error, simply ftp into the root of the site and delete the file. It’s hidden, so make sure your ftp software isn’t suppressing the ability to see a hidden file.
Whatever Plug-in or update failed likely also broke something. Usually, if it’s a Plug-in then you’ll need to re-install that plug-in, as the update process removes the old Plug-in and then adds it back. If it’s a Theme, you might need to re-install the Theme.
Programmatically, you can also enable Maintenance Mode by creating this file and then disabling Maintenance Mode by deleting (or renaming) the file again.