Over the last week many people’s copies of Microsoft’s Windows 10 Pro Operating System have started to report that it is not activated.
This seems to have been related to an issue with Microsoft’s Activation Servers and they have acknowledged the problem and have released a fix.
According to Microsoft running the Troubleshooter should now fix the problem.
More information on the issue can be found in this article:
https://www.bleepingcomputer.com/news/microsoft/microsoft-bug-is-deactivating-windows-10-pro-licenses-and-downgrading-to-home/