If Windows 10 is suddenly greeting you with a message that your operating system is not activated, there's no need to worry as it appears Microsoft has messed up again. A support topic on the Microsoft Community reveals the software giant has a temporary issue with its activation server. This causes perfectly fine copies of Windows 10 to no longer be recognized:
Microsoft has just released an Emerging issue announcement about current activation issue related to Pro edition recently. This happens in Japan, Korea, American and many other countries.
I am very sorry to inform you that there is a temporary issue with Microsoft's activation server at the moment and some customers might experience this issue where Windows is displayed as not activated.
Our engineers are working tirelessly to resolve this issue and it is expected to be corrected within one to two business days.
Microsoft recommends to sit tight and do nothing. The activation issue should be resolved within one to two business days.