Description of error
If you’ve been keeping on top of your WordPress updates using the automatic upgrade feature, you may have run into a problem immediately after upgrading. A message that prevents access to any page on your blog, “Briefly unavailable for scheduled maintenance. Check back in a minute”.
Cause of this message
During an automatic upgrade, WordPress places a file in the blog root directory called “.maintenance” to prevent visitors from being confronted with ugly, broken pages during the upgrade process. That’s a great little built-in feature, but if the upgrade gets interrupted or fails for any reason, that file doesn’t get deleted and nobody, including you, can access your blog. No worries, this is (most likely) the simplest fix ever.
Official Solution
All you have to do is delete the .maintenance file from the blog root directory. Simple as it gets. After you’ve deleted the files you should be able to access the site just fine, but you may have to run the automatic upgrade again. How do you know if you should run it again? You’ll see a notification on the dashboard page that a new version of WordPress is available…same as you always see when a new upgrade is available. If you don’t see that message, you’re good to go.
In case problem persists
In my case the “. maintenance” file was not there at all. Started exploring the blog files and there it was.
In wp-content/upgrade/<pluggin_name>.temp
The pluggin name was the latest pluggin i tried to update. So i deleted this folder and everything came back to normal state.
I hope this helps.