Urgent-Invoice status: "Unpaid

My invoice status still shows "Unpaid" 5days after the date of payment and is now overdue. I have been looking for Booking.com's contacts that I can reach from Namibia but to no avail. Since this is my first invoice, I guess it is why my inbox shows "you have no messages". There is no option to initiate an email in my inbox. Please help as I want to believe that my property might have been closed due to the "overdue'' account which was actually paid. I have not been getting any booking inquiries. Where do I send the proof of payment to? I have even tried to reach them by objecting to one invoice just so that they can respond but no one has responded till now. Thank you, anyone Lindiwe