There is an ongoing issue about Windows 10 suddenly being shown as deactivated. Affected users do not need to do anything as it is a known issue. Here is what was said by Microsoft Chat support:
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