Today I was trying to upgrade my wordpress installation to latest version and the upgrade failed and when I try to refresh the page or go back to wp-admin I get the following error

Briefly unavailable for scheduled maintenance. Check back in a minute

I’m sure you have had the same problem and that is why you were looking on Google to find a solution.

you’re getting this error because wordpress upgrade was not complete and wordpress did not delete the auto generated file called (.maintenance) in the root directory of your wordpress installation

Lets Fix it :

Using an FTP client (I Use Filezilla) Login to your account and in the root of your wordpress installation, Find a file called .maintenance and delete it, see the image below

When done, refresh your web browser and you should see the page you were expecting