Briefly unavailable for scheduled maintenance. Check back in a minute.
升级时没有备份数据库，出现这个时立马就faint了

网上查了google，登陆ftp删除了根目录下的 .maintenance 文件  就ok了。

Cause of the Maintenance 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 on Earth.

Removing the Maintenance Message

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.

Of course, this fix assumes the blog upgrade failure was really just a one-time occurence and not a symptom of a larger problem. If you’ve deleted the file and the automatic upgrade continues to fail, you’ve probably got a larger issue at hand, and that’s a topic far too long to cover in this article.