Solution is
The error is occured because of failure on Wordpress version upgrade. Try a manual update.
-    Download the new version Wordpress zip file and extract it.
-    Then take a back up of your Wordpress.
-    After that rename wp-includes and wp-admin directories to wp-includes.bak and wp-admin.bak.
-    Then upload wp-includes & wp-admin directories from extraced folder to your web host using the FTP.
-    Upload the files from new wp-content to your present wp-content folder. Replace and overwrite the existing files.
-    Upload the new versions rest of files from the root directory to your existing Wordpress root directory on the server.
-    Now remove the .maintenance file at your Wordpress directory using FTP.
-    Login to WordPress admin and you will be given a link URL like "http://domain.com/wordpress/wp-admin/upgrade.php". Just follow the link and instructions for further steps.
-    Now clear your cache to see the changes if you have caching enabled.
A simple tip by
Ultimatecine.com
