How to: Fix WordPress Error 'Briefly Unavailable for Scheduled Maintenance'
Refresh the page, clear the browser cache; no change. Here's how to fix it.
Essentially what's happened is that WordPress site fails to finish the update, leaving the whole site stuck in maintenance mode. WordPress won't let you view or edit anything until all maintenance tasks are flagged as complete.
Cause and Effect
The maintenance mode page is not strictly an error, but a notification that you should get for the duration of the update process. While WordPress is downloading, extracting and installing updates, it locks all content changes in the database. Updates finish, it unlocks. The maintenance mode notification is controlled by a temporary .maintenance file in the website’s root folder. When updates are completed, the .maintenance file is deleted and the site re-enabled.
Occasionally, server issues such as load, response times, or memory issues prevent the update process from completing. Updating conflicting plug-ins or new versions incompatible with WordPress core can cause an abnormal state in the update process so that it doesn't finish.
I also discovered if you click out of the update screen in the WordPress admin area, you can interrupt the update process such that it doesn't finish all the updates OR doesn't delete the .maintenance file. In these cases, you need to manually delete the .maintenance file and complete the updates.
Delete the .maintenance file
You will need FTP access (or file manager access on some hosts) to the root folder of your WordPress directory. Login to the server, got to the site's root folder and locate .maintenance. This is a hidden system file so you will need to make sure you have 'view hidden files' enabled in the FTP client or file manager.
In Filezilla, the show hidden files option is under Server, Force showing hidden files from the menu bar.
You just need to right click or select this file and delete it. In Filezilla, right click and select delete from the context menu.
Now that your site is out of maintenance mode, go back to WordPress updates to check all the available updates have completed. Depending on how or when the update process halted, you may have to re-select and trigger updates for some, all or none or the updates available. If the update queue was interrupted, there may be some complete and some outstanding. It may be that only the final step of deleting .maintenance failed leaving you stuck in maintenance mode and you're good to go having taken care of it. RC