Remember when Microsoft gave that FREE upgrade to Window7 and 8 people to Windows 10. Well when you did the free upgrade it never provided a license code. The disk on that computer just died and I never followed your image backup tips, so now I can only install the original license. I will have to buy a Windows 10 license. That isn't fair. What's up with that?
Yes this is why Bill Gates is so rich. They force you into buying another license. Actually there is a way and for $10 Ken I will tell you.
When you upgraded you needed to enter your email address that you used when upgrading. It may be a MS email like joe@hotmail, your own domain email, or even a gmail or yahoo address. It is referred to at MS as your MS Account address.
So get a copy of your Windows PRO or Home install CD or image, install it on the new disk in the computer, and when it asks for the product key at initial install, don't enter a key. It will continue the install and after it is installed you will have 2-3 days to activate the license.
At that point enter your "MS Account information and wala it will license the Win 10 system. Isn't Bill great! https://support.microsoft.com/en-us/help/12440/windows-10-activation