Money gone and no response from Booking.com!

I booked a hotel in San Francisco through Booking.com and was overcharged on arrival by the Hilton Berkeley. I was assured that the amount would be paid back into my account 4-6 days after check-out, this didn't happen. Since then (10 Dec 2019) I have been trying to resolve this through booking.com with no response from anyone. Terrible service!