Does supplication change a person's decreed destiny, and what becomes of a supplication that is not answered?

Supplication is part of Allah Almighty's decree, and there is no contradiction between them. Allah decrees things with their causes. Just as He decreed satiety with eating, He decreed provision with supplication. It is not permissible to refrain from supplication on the pretext that destinies are already written. Supplication is a cause, like medicine. The Prophet, peace and blessings be upon him, supplicated for people to have many children, provision, and guidance.

As for the supplication for which an answer has not been decreed, it will either be stored for the supplicant on the Day of Judgment, or an equivalent amount of evil will be averted from him. For a supplication to be answered, there are conditions, including sincerity, that it does not involve sin or severing ties of kinship, that the supplicant's food and clothing are lawful, and that he does not rush the answer.