This is not typical, but can occur after a BIOS update when one or more hardware identifiers changes. Typically 3 or more changes triggers the reactivation request. In any event, you can reactivate (legitimately) using your existing key. Just go through the activation process using the internet.
If that fails, call and the automated activation system will guide you through the process. I suggest a pen and paper for this method. If you have Office installed, it might need to be reactivated as well.