Maybe you purchased a licensed Windows 10 laptop and formatted it all and installed Linux. Is your license lost?
Well, no. As the license key isn't stored in hard disk like in the old days, rather it's stored in firmware.
You can read it back any time running the following command in Linux.
sudo strings /sys/firmware/acpi/tables/MSDM
And if you reinstall Windows, the installer will read it back from there and won't ask you for a new key.