The document (promissory note suspected) is a valid document even if not dated since it is signed by the person who owes you the $40,000. I would write the person a note asking for payment beginning the first of Spetember 2012 and continuing until the obligation is paid off. Keep a copy of the letter for future use and need.If you do not get a response as to what you want, then you should consult with an attorney who does contract law in the county where you reside for guidance in the matter. The lawsuit can be filed in the county where you reside if the agreement was entered into there. As to costs for an attorney, that issue is best left to the person who you engage in your matter.