Solution is simple just you remove the .maintenance file

A file named .maintenance will be placed in your wordpress site during an automatic upgrade to alert viewers that the site is under maintenance. If your upgrade fails or is corrupted, then the file will remain in the Wordpress folder and it will not get deleted.

For removing the .maintenance file, follow the below steps

•     Locate your blogs root directory using FTP.
•     Open the ".maintenance" file from the root folder.
•     Then finally remove the .maintenance file from the blogs root directory.

Once you solve this problem, you need to run the Wordpress automatic upgrade again. Now the problem is fixed.

A simple tip from
Ultimatecine.com