Easy put WordPress in Maintenance Mode.

wp-maintenance-mode.php

/*
* put this on the very bottom of the functions.php file of your theme
* or create manually .maintenance on your web root with desired message
*/

// Activate WordPress Maintenance Mode
function wp_maintenance_mode(){
  if(!current_user_can('edit_themes') || !is_user_logged_in()){
  wp_die('<h1 style="color:red">Website under Maintenance</h1><br />We are performing scheduled maintenance. We will be back online shortly!');
  }
}
add_action('get_header', 'wp_maintenance_mode');