Earlier today I updated a few WordPress plug-ins. After the updates were finished, the site disappeared off the net and all I saw was a ‘Briefly unavailable for scheduled maintenance. Check back in a minute.’ message. Yikes!
There are plenty of pages on the web that describe how to fix this: There is a .maintenance file that is placed in the blog base directory during WordPress upgrades. If the upgrade times out or something goes wrong, this file does not get removed at the end of the process. As long as it exists, a maintenance message will be displayed.
To get your site up again, log in with an FTP-client and delete this .maintenance file. It is a hidden file so by default your FTP client may not display it. In Filezilla, you can use the Server > Force showing hidden files menu option to display hidden files. This did not work for me, the .maintenance file was nowhere to be found. I started to panic and then luckily thought of refreshing the FTP-window. Hey presto, the .maintenance file became visible, I deleted it and my site was available once more.
Don’t forget that this happened because the upgrade did not finish successfully. You may have to run it again.