Here is solution of this problem when you activate wp mu on your single domain i.e. you add subdomain through WordPress MultiSite suddenly your pages are not showing up istead it through ‘404 Not Found’ Error…
First thing you have to do is navigate to:
Network Admin > Settings > Network Setup > Then copy given appropriate code to .htaccess & wp-config.php file reaspectively.
& your site should be working peacefully now.
Hope it helps,
Thanks & Regards