If you have recently done some upgrade work on your WordPress site, and you cannot login due to the following message, I have the fix for you.
Briefly unavailable for scheduled maintenance. Check back in a minute.
What Causes This Problem
This problem is caused by a failed automatic update on one or more parts of your WordPress site.
When WordPress is automatically updating the core WordPress files, plugins or themes, your site is marked as under maintenance.
This problem occurs when the under maintenance is not marked as done at the end of the process. This can be caused by an interrupted update, the updating taking so long that it times out or browser crashes.
How To Fix It
Fortunately the fix is fairly simple, you need to attach to your site using ftp or with the file manager in your hosting account, and in the root of your installation you will see a file called .maintenance, simply delete this file and you site will come back to life.
SPECIAL NOTE: files which start with a full stop/period are marked as hidden files on unix/linux, you may need to enable show hidden files on your FTP client before you can see this file, for example on filezilla you need to go to server->force show hidden files.
If At First You Don’t Succeed
Once you have deleted the .maintenance file it is a good idea to re-apply the updates you were doing to make sure they have been completed correctly.