If you bought a retail version of Windows 10, you should
have gotten a product key. If you have mislaid it, you need to
contact Microsoft with a proof of purchase.
If you instead upgraded to Windows 10 from an activated Windows 7/8/8.1, you will not have a product key. Instead, the PC gets registered on Microsoft activation servers with a digital license (formerly called digital entitlement) that will always be used to activate Windows 10 on that PC.
However, you may link your Microsoft account to that license on your device. This way you may reactivate Windows by using the Activation troubleshooter if you make a hardware change later, such as replacing the motherboard, just by logging into your Microsoft account.
Using a recent Windows 10 release,
you can now use the product key from Windows 7/8/8.1 that was previously used to activate Windows on this same device,
by going to
Settings > Update & security > Activation and selecting
Change Product Key, and you can do this even for a clean install of Windows 10.
See the article
How to link your Windows 10 product key to a Microsoft account.