Quite recently while I was upgrading one of my blogs using the inbuilt WordPress upgrading option, I assumed everything would go fine, however when I went to blog I kept seeing a message saying Briefly unavailable for scheduled maintenance. Check back in a minute..
Since I was doing a upgrade I thought this message would go away shortly, however it did not.
Well I did a bit of research and found that the above error is because of a maintenance file that WordPress adds to the root directory whenever it is auto-upgrading.
Most of the time it deletes the maintenance file, however sometimes it may not be able to do that and users will see the above message when they visit your blog.
To fix this error you will need to login to your FTP server and search for a file called .maintenancewithout the quotes, this file should be in the root directory of your WordPress installation.
Once you find the file delete it from your FTP server. Once you do that your blog will return to normal, hope this solves your problem too.