Put Your Site in Maintenance Mode Using Function.php in Wordpress :

How To Display Your Site in Maintenance Mode. Put below Code in Function.php file.

function wp_maintenance_mode(){
    if(!current_user_can('edit_themes') || !is_user_logged_in()){
        wp_die('Maintenance, please come back soon.', 'Maintenance - please come back soon.', array('response' => '503'));
    }
}
add_action('get_header', 'wp_maintenance_mode');

  And Save your File. Than See Your Site Home Page Give Your Maintenance Message.