Check that the web server user (e.g. apache, www-data, php-fpm) has access to read config.php (in the Moodle source code folder).
If that doesn't work, try adding the debugging lines to config.php which should show an error.
If that doesn't work, find the web server error logs and see if they show any error.
If that doesn't work, please tell us more about the environment, e.g. web server operating system, PHP version.