Maintenance mode for the admin only.txt

function admin_maintenace_mode() {
    global $current_user;
    get_currentuserinfo();
    if($current_user->user_login != 'ADMIN_NAME') { ?>
                        <style> .updated{margin:30px !important;} </style><?
                        wp_die('<div id="message" class="updated"><p><b>Maintenance mode:</b> We are currently making updates. Everything will be online shortly.</p></div>');
                }
}
add_action('admin_head', 'admin_maintenace_mode');