This simple and lightweight
WordPress maintenance plugin puts the site into maintenance mode by sending a '503 Service Unavailable' status to all unauthenticated clients.
Thus it effectively inhibits search engines from both losing your site's existing content and indexing your maintenance page as well, so your site will not lose its rankings while it is out of service.
Content will even be hidden from consumers of the site's RSS or Atom feeds.
Please refer to the
WordPress Maintenance Plugin article for additional information.