Do I still owe a debt to a deceed person?

Yes, you still owe the debt even though the lady ped away. The lady's estate is now the creditor. If you do not pay, the estate's executor/administrator can file a lawsuit against you. Since you did sign a promissory note the estate will definitely win a judgment against you. One thing I am curious about though is why you would even ask this question. Why would you think that you no longer owe this debt? Please do the right thing and take care of your debts.