WordPress makes upgrading very easy . You simply click "Update now", wait for a minute or two and your system is up to date. If, well if everything works fine.
The most common problem during an upgrade is the Internet connection to drop unexpectedly or the user to shut down the browser unintentionally. In both situations the upgrade will stop instantly.
If you try to log in to your backend again you will receive the message
"Briefly unavailable for scheduled maintenance. Check back in a minute."
This message is useful to keep users away from your blog during the upgrade but right now it's keeping you from restarting the upgrade. To solve this problem we have to take a look at how WordPress determines that it's in maintenance mode.
WordPress is looking for a possible maintenance mode very early to prevent the system dying from any fatal error. So it using a very simple method by writing a file called
.maintenance to the WordPress root directory. If the blog or backend is accessed it will check for the file and stop if it's present.
Knowing this the solution to our problem is quite simple: access your WordPress system via FTP and delete the file
If you're not able to log in to your server via FTP for some reason there is a second method for escaping from maintenance mode: simply wait 10 minutes!
.maintenance contains a timestamp of the time the file was created. If this time is less than 10 minutes ago WordPress will go into maintenance mode otherwise it will continue to work as usual and enable you to restart the upgrade unless something worse keeps it from starting.