If you need to go into temporary maintenance mode and close the site to visitors in the meantime, you can enter the below code:

Turn on maintenance mode WordPress

/*
* http://blog.templatetoaster.com/15-useful-code-snippets-for-your-wordpress-site/
*/
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');