If you have Windows 8 installed on a computer already you can retrieve it.
There is some software you can use to get your product key back.
If you don't have it, the only way to get a key is to remember which credit card you used to purchase it and you can call to get your product key from Microsoft.
Do you have Windows 8 installed already?
yes but there is a proplem unable start up
Now you are trying to re-install?
i also tried to install the disc but requires key number
The only way to get the key is to call Microsoft and provide the exact credit card you purchased it with so they can find you in the system.
Unfortunately, once you don't have a key, those are the only 2 ways to get the key back.