Dear valkyre,
Hello! Welcome to the Microsoft Community. I'm glad to be able to assist you. If I misunderstand your question, please let me know promptly.
I understand that you were suddenly told that Windows is no longer activated after logging in one time, and you clicked "Change Product Key" and pasted your current license, but it failed. I understand the trouble this problem has caused you, and I will try my best to help you solve it.
This problem seems to happen frequently recently, and many users have reported the same problem as you, and all of them used OEM keys.
Windows 11 deactivated after 24h2 update - Microsoft Community
I think this problem may be related to the latest 24H2 update. It is not clear why this happens. Let's try the conventional solutions. If they don't work, we may need to wait for official updates.
Try the following methods to see if it helps you:
-Search for "cmd", open the "Command Prompt" as an administrator, enter the following and press Enter:
slmgr /rilc
Open the activation page in the Settings app and select the Troubleshoot option. You may need to run it a few times to see what it says?
- Try to find your original OEM key and re-enter it to activate Windows. You can ask your manufacturer, the OEM key should be imprinted on the device hardware.
About the error code during activation:
0xC004F213: Windows reported that the product key was not found on your device.
0x803fa067: The activation server cannot process the request
Get help with Windows activation errors - Microsoft Support
I hope the above information can help you. If you have any questions, please feel free to contact me.
Best Wishes
Pyke.D | Microsoft Community Support Specialist