Since your 25-character product key pairs your license with the hardware of your computer, changing/upgrading any major device such as the motherboard or processor on your computer breaks the pair. This makes Windows believe that the product key that was originally registered to a different computer is now being re-used on a new one, unethically.

Because of this, Microsoft temporarily suspends your Windows’ activation, and you must perform some steps to reactivate the OS.

Here are they:

Step 1
Get to the Activation window

Click Start, click the Settings icon, click the Update & Security tile from the Windows Settings window, and click Activation from the left pane. This takes you to the Activation window. If you’ve recently changed your motherboard, processor, or both, you can see a message in red in the right window informing that Windows needs reactivation.

Step 2
Begin troubleshooting (sign-in to your Microsoft account)

Click Troubleshoot, wait while Windows detects any issues with the activation, click I changed hardware on this device recently on the Troubleshooting has completed window, and enter your Microsoft account’s username and password in their corresponding fields when prompted. This signs you in to your Microsoft account, and takes you to the Reactivate Windows 10 window. Here you can see a list of all your devices that share a common Microsoft account.

Step 3
Reactivate Windows 10

Scroll down the Select this device from below list, check the This is the device I’m using right now box under the computer with Windows 10 you’re trying to reactivate, and click Activate. This checks your product key with your computer hardware and Microsoft account, and attempts to reactivate Windows 10 automatically. Most of the time this process is successful, and you can continue using your activated OS upon completion.

Step 4
Re-enter a product key (if troubleshooting fails)

Click Change product key from the Activate Windows now section on the Activation window, enter the product key in the Product key field on the next box, click Next, and continue following on-screen instructions from there. This checks with your product key, and attempts to reactivate Windows 10. This method works and is most helpful if you upgraded from Windows 7 to Windows 10, and you already have a genuine Windows 10 product key.