Hi, let me ask you a few questions to narrow this down.
First, where does it show up: a Windows toast in the corner, a pop-up inside a specific app, or a banner in your browser? What does it say, word for word, including any error code or button text?
Does it appear at a particular moment, like at startup, on sign-in, or when you open Office or OneDrive, or is it random? Also, did anything change around when it began, such as a Windows update, a new Microsoft account or password, a hardware swap, or a reinstall?
Is this a personal PC or one managed by work or school? If it's managed, the device may have dropped out of Entra ID or Intune. And does Settings > System > Activation say Windows is activated, and is it linked to your Microsoft account?
Finally, does the notification ask you to call a number, click a link, or download anything? That would point to a scam rather than a real Windows alert.