Booking.com's FAQ pages can be found here.
You'll find help on the following topics:
Making a booking
Changes and cancellations
Guarantees
Payments
You can also email them at customer.service@booking.com
Or call them as below:
UK (local number) +44 20 3564 6725
Australia (local number) 1800 058 744
North America - English 1888 850 3958
North America – Spanish 1866 938 1297
International (English) +44 20 3320 2609