Making changes to your hardware or operating system after initial activation of a license can cause the license to not recognize the current system. This can happen, for example, when changing the OS user, as the machine identifier changes.
If you've lost the product key and need to recover it, contact the distributor who sold you your system, if possible. They should have the old key available. Obseron support may also be able to figure out the key if you provide the activation ID, as well as help reactivate the license.
In case you have other changes planned for your system than the ones you've already made, such as hardware or user changes, these should be done before reactivation of the license as it will bind it to the current system.