If you get this message when upgrading WordPress, no worries.
WordPress put a .maintenance file on your blog’s root directory when you use automatic upgrade. This was meant to protect your blog visitors to see the ugly face of your blog while upgrading. However, when the upgrade fail (happen at times), WordPress don’t delete the files automatically. Then everyone, including yourself will unable to see your blog, and instead seeing the message “Briefly unavailable for Scheduled Maintenance. Check back in a minute.”
What you need to do if you’re facing this situation, is to delete the file manually.
1. Access your blog folder through FTP. Make sure you enable to view hidden files.
2. Find .maintenance file, and delete it.
3. Refresh your browser and done 😀