You should only be liable if you acted tortiously, or wrongfully, personally--that is, if you personally did some negligent (unreasonably careless) or intentional wrongful act. You should not be liable simply because you were the registered agent. You need to ascertain the basis of the judgment to see if it may be valid. Another possibility is if they named you in the lawsuit and you never responded, they may have obtained default judgment (like forfeiting or losing a ball game by never showing up) against you; in default judgments, it's all about not responding, so the court never gets to the substance of the matter to see if it's actually warranted. If this happened but you never received notice (the official papers) of the lawsuit and can prove that you never received notice, you may be able to have the judgment set aside or vacated.