Yes, please don’t downgrade like that. It’s not good unless you just have a backup to restore from.
Try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.
That will bring you back up to WordPress 4.0.1.
If you see the issue again, let’s try to fix that specifically. Try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
If that does not resolve the issue, try switching to the Twenty Fourteen theme to rule-out a theme-specific issue (theme functions can interfere like plugins).
I have been able to bring back the website but unable to login to the backend. How can I fix it.
Try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.
If that does not resolve the issue, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).
