If you would like to enquire about a refund or anything to do with your invoice, please contact the booking site directly. They are responsible for the booking and payment process, so will be in the best position to help you. As a metasearch, trivago is not directly involved in the booking or payment process. Your booking confirmation will include their contact details.
If you are seeking a refund and believe it was due to an error on trivago, please for more information.