This issue is due to some core files in wordpress being corrupt.
The solution to this issue is to download new version of wordpress and upload new files and folders mentioned below
wp-includes , wp-admin and below files
wp-activate.php
wp-blog-header.php
wp-comments-post.php
wp-config.php
wp-config-sample.php
wp-cron.php
wp-links-opml.php
wp-load.php
wp-login.php
wp-mail.php
wp-settings.php
wp-signup.php
wp-trackback.php
xmlrpc.php
This resolved the issue and wordpres site was working fine