Unable to activate Windows 10 after upgrade

[highsenberg]

Hi, I have a Lenovo Z500 laptop which came with Windows 8.1 Single Language License.

Yesterday I upgraded to Windows 10 via Windows Update. But since yesterday it says "Windows not activated". When I click on the Activate Windows icon and click the error details link I get the error code: 0xC004C003.

Does anyone know how to solve it? I know some people facing the same issue but they did a clean install, I didn't.