Post this question in the contracts section.
Please note that this answer does not constitute legal advice. It is merely intended to provide general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.
You should write a letter by certified mail, return receipt requested to Booking.com and the hotel to set out the facts and to request that you be issued a refund. When the company refuses or ignores you, dispute the charge on your credit card, and use your letter as evidence.
If the reservation was made well in advance so that there would be no lost room rental before your cancellation, Booking,com has a hard time justifying its charge while providing nothing in return.