In Windows 10, Microsoft introduced new rules for activation - the key is attached to the hardware of the device, and if the PC is replaced, for example, with the motherboard (or processor), the activation may fail. However, this does not mean that you need to buy a new key - several times you can re-activate the system with an old key, but for this you had to go to this PC in your Microsoft account with a key attached to it before replacing hardware.

To reactivate the system after replacing the hardware, click Start> Settings> Update and Security> Activation and select Troubleshooting. In the troubleshooter, select "I recently changed the hardware on this device" and click "Next". Now log in to your Microsoft account and select from the list of devices connected to it the computer that you are currently using, check the box next to "This is the device that I'm currently using", and then click "Activate". After that, the system will be activated again.