When you power up a new system Microsoft gather information about the system and see if it matches known systems. If it does then it activates automatically. If you bought your copy of Win 10 then it's remarkably tolerant, in that you can change a lot and Microsoft will still activate your copy of Windows. But if you go too far, change too much, or you didn't buy your copy of Win 10 ( you upgraded for free ) then you might find it will not activate. At that point you need to enter the key, which may be a problem if you never had one. There is a utility I believe that is called the Key Finder Utility. I have no idea whether it will find a key on a product that is not activated. You may, I am afraid, find you just have to buy a new key. Microsoft has no sympathy at all for people who lose their keys.