If you are doing an automatic upgrade with WordPress and the only thing you see where your site should be is the message, “Briefly Unavailable for Scheduled Maintenance. Check Back in a Minute.“, your upgrade failed. I have seen this message before and knew what to do, but it just happened during an upgrade to WordPress 3.5.1 so I thought I would let others know about the issue and the fix.
When WordPress starts the automated upgrade process, it places a file named .maintenance in the directory where your site is installed. The file contains a snippet of PHP code that triggers the display of the message. The upgrade process normally only takes a few seconds to complete, so most site owners never notice the message. When the upgrade has completed, the file is supposed to get deleted, but sometimes that does not happen.
The fix is to simply delete the .maintenance file, which releases the site. After doing this, check the dashboard page in the admin to see if WordPress registered the new version. Sometimes the upgrade process hangs up, other times it runs into an error along the way. There are also incidents where WordPress simply fails to delete the maintenance file. I have heard of a few cases where the all the pages were all white after running an upgrade.
Even if WordPress acknowledges the update, it may or may not have completed properly. Whenever you get the maintenance message it is always a good idea to download the current version of WordPress and use an FTP utility to manually copy all of the WordPress files to the site.
If for some reason you made changes to one of the default WordPress themes and did not rename the theme as you should have, do not copy a new version of the theme to the themes directory because you will loose all of your previous changes. When ever a theme, such as TwentyTen, TwentyEleven or TwentyTwelve is modified, it should be renamed to prevent any overwrites.