The hardisk where I install RouterOS level 6 has gone bad. How can I get my RouterOs back? I have the ProductID and the License with which it was created.

I hope you must be having latest backup else you have to configure everything again...

Install new hdd & download mikrotikOS same version from http://www.mikrotik.com/download.html

if you want to boot from cd download .ISO file & mount it to make copy disc.
else you can do a net install
http://www.mikrotik.com/testdocs/ros/2. ... nstall.php

after installing when you login through winbox it will ask you for license key.