You can absolutely travel without their permission. Either physically find and take your passport back, or repoprt as stolen and apply for a new one. You don't need parent's permission. Whether you do so or not is another matter.
A letter of consent, preferably in English and notarized, from the other parent or signed by both custodial parents. The letter should say: "I acknowledge that my child is traveling outside the country with the name of the adult with my permission."
The letter should include the other person's contact details and details about the trip. It also helps if you've: evidence of your relationship with the child, for example a birth or adoption certificate. a divorce or marriage certificate, if you are a single parent but your family name is different from the child's.