Any number of things could be the cause inclusive that you didn't 'set' the OC properly, to you damaged the card and now is 'locked' at 1Kmhz. Why not look up the card online and see if you see other OC warnings and symptoms for it. It would only be 'Afterburner's fault' if you set Afterburner to apply settings to it. Simple test, uninstall Afterburner, can you OC again?