A strange bug in Microsoft's activation servers is causing deactivation of several Windows 10 Pro licenses and prompting users to install Windows 10 Home. Trying to purchase a Pro key from the Store is apparently not solving the problem. Fortunately, Microsoft is aware of this issue and said that it is working to resolve it at the earliest.
https://www.notebookcheck.net/Some-Windows-10-Pro-licenses-are-reportedly-getting-deactivated-to-Windows-10-Home.357981.0.html
It literally just happened to me while I was reading this article on my Dell Windows 10 pro workstation
Happened to me. Still waiting for a fix. :(
This also happened to my pc and I just fixed it by reentering the activation code for my Windows 10 pro. It seems to be working fine.