If you sold by owner and it is the buyer of your house in question you need to conduct further due diligence. You need to get that buyers lender information and consult with them immediately. The lender should provide a loan status update. There are likely many questions to go over so re-read your contract for contingencies, assure they are met and talk to the buyer's representation as well. You can issue a cure notice if that is practiced in your state and give notice to perform. You may want to consider a per diem if they are asking to extend as well. It's hard to help without reading the contract but seek help of an attorney if you aren't able to get further on your own.
Best of luck