Missing product key or digital license
Windows 10 activation requires either a 25-character Windows product key or a Windows digital license.
Find out which activation method your device uses and where to find it.
A different product key or version of Windows was used during repair
If your PC was repaired or rebuilt, it's possible that a different version of Windows was installed. Or, if a different product key was used for your PC during repair, that key might have been used on more PCs than allowed by the Microsoft Software License Terms.
If Windows was activated before your PC was repaired or rebuilt, reinstall your original version of Windows and re-enter the product key included with your PC or original copy of Windows.
One copy of Windows was installed on multiple PCs
If you have one copy of Windows and you installed it on more than one PC, activation might not work. The product key has already been used on more PCs than allowed by the Microsoft Software License Terms.
To activate new additional PCs, you must buy a new product key or copy of Windows for each one.
Activation works with genuine copies of Windows. A counterfeit is a copy that wasn't published and licensed by Microsoft.
The Microsoft How to Tell website can help you determine whether your copy of Windows is genuine or counterfeit. If it is counterfeit, you must buy a new copy of Windows.
A pre-owned PC
If you bought a PC with Windows already installed, it's possible that the seller used the product key on more PCs than allowed by the Microsoft Software License Terms.
Ask the seller for the original Windows DVD and product key. If the DVD and product key aren't available, or if you tried them and still can't activate your PC, you must buy a new product key or another copy of Windows.
If you made substantial hardware changes to your PC, such as replacing your hard drive or motherboard, Windows might no longer be activated. For Windows 10 (version 1607 or later), you can add your Microsoft account to help you reactivate Windows 10 after making a hardware change.