I had a machine that randomly decided that it was no longer activated. I have heard about some Microsoft server issues with activation so I gave it a few days and a few restarts to see if that would help. It didn’t.
I went into Settings > Update & Security > Activation but I had no option to activate. I waited another day.
I decided I would get updates just in case Microsoft pushed some sort of a fix for this issue, but that did not resolve the issue. It did, however, give me an option to Troubleshoot Activation Errors and that did the trick.
Settings > Update & Security > Activation: There was a button to Troubleshoot Activation Error. I selected that, and it went out poked the server and reactivated my version of Windows.
I have heard that some people need to call into Microsoft to fix this problem.
Here is a link if you need to take that step: https://support.microsoft.com/en-us/contactus/
Hopefully, you will find that a Windows Update and Troubleshoot Activation Error will do the trick.