From time to time I want to log down the WordPress backend from Users who update the content in the backend.

Just add this code in Functions.php

function wp_ob_backend_offline() {
 if ( is_admin() )
 {
 wp_die( '<strong>Site offline for maintenance!</strong>
Hang on - we´ll be right back online!', 'Site Offline For Maintenance', '');
 }
}

add_action('init', 'wp_ob_backend_offline');