My parents sold a business with all debts included. A contract was signed and notarized. Who is responsible for past taxes and debts?

Your question is unclear and no one can answer it in this form. It is possible that both the buyer and the sellers are liable for debts of the business. Among other things, in order to be able to answer the question, an attorney would need to know the terms of the purchase contract (i.e., whether the buyer assumed these debts), the nature of the debts (e.g., taxes, accounts payable, etc.), whether the seller guaranteed the debts personally, and whether the buyers have a good reason for failing or refusing to pay. You need to see an attorney about this -- it's not the kind of question to which you can receive a reliable and comprehensive answer on the internet.