You don't state what kind of a debt  it is, but if it is one pursuant to some contract or other written agreement (including a loan; a credit card debt; or debt for medical care), the creditor can still seek payment from you up to four year after you failed to pay money when money was due. Therefore, based on what you write, the creditor could still sue you for this money; if you don't respond to their lawyers within the time frame they indicate,  they will presumably then institute a law suit against you. A good way to start is to ask the lawyers to provide proof or documentation of the debt; once you have it, you can decide if you believe you actually owe the money or not. That will inform your decision as to what to do: e.g. pay it; try to settle it for a lesser amount; refuse to pay and if necessary defend against it in court; etc.