Post not redirecting to non www after changing from www

I did it through cpanel

That’s certainly part of the problem. cPanel will add the redirect at the end of the .htaccess file (after the WordPress front-controller) – so they are not getting processed. You need to manually edit the .htaccess file to move these directives to the top.

However, that doesn’t entirely answer your question as this wouldn’t affect only “some” WordPress pages.

Accessing your site with the www subdomain should still show the relevant page – not a blank page?!