Solution:
Just try following steps:
- Make a backup of your WP installation.
- Download a new WP from http://wordpress.org/latest.zip, if you want an old version download from https://wordpress.org/download/releases/
- Replace all other files except wp-content folder and 2 files; .htacesss, wp-config.php
It should work. if you still find any error please reply the same thread.