How to clear the “Briefly unavailable for scheduled maintenance” message after doing automatic upgrade?
As part of the automatic upgrade WordPress places a file named .maintenance in the blog base folder (folder that contains the wp-admin folder). If that file exists, then vistors will see the message Briefly unavailable for scheduled maintenance. Check back in a minute. To stop that message from being displayed to vistors, just delete the .maintenance file. The automatic upgrade should be executed again, just in case it failed. Note the core automatic upgrade feature was added with Version 2.7.