If you are going to make changes in your WordPress e.g you are going to test new theme or other making changes in WordPress code, or try a variety of plugins. You make mistakes and hope that no one is viewing your site when you’ve temporarily trashed it.

After numerous temporary and minor disasters I now use the maintenance mode WordPress plugin.

Once maintenance mode is activated, a page will go up saying that your site is undergoing maintenance and to try back in a hour. If you are logged into your site, you will still see the site normally and the changes you are making while your visitors will see the maintenance page.

You can get Offline Mode plugin form here