You will need to contact the booking site you booked the hotel with to change or cancel your reservation.
To identify which booking site you used, you should be able to find its name on your booking confirmation email. Please double-check your spam folder should you not have received one.
If you cannot find your confirmation email, you can always search on trivago for the hotel and dates you booked to identify the booking site you made your reservation with. You may also be able to find more information by looking in your internet browser history for the day you made the booking.
You can also try checking your credit card or bank statement, as the name of the booking site you made your reservation with may appear next to the outgoing payment.