What should one do who has stolen, consumed the unlawful, and committed adultery, and then repented, but is unable to repay the numerous debts to their owners, some of whom are unknown, and is their prayer worthless due to not repaying these debts?

Allah forgives all sins for whoever repents with sincere repentance, even polytheism, and He replaces the repentant person's bad deeds with good deeds.

To repent from adultery and sins that do not involve the rights of other people, one must seek forgiveness, feel remorse, abandon the sin, and resolve not to return to it.

As for the rights of other people, they must be returned to their owners or their heirs, or given as charity on their behalf if they cannot be reached, with the option later for them to either accept the charity or take their money.

Whoever is unable to repay a debt but intends and resolves to do so, there is no sin upon them, as supported by the hadith: "Debts are of two kinds: whoever dies intending to pay it back, then I am his guardian. And whoever dies not intending to pay it back, then that is the one from whose good deeds will be taken, for on that day there will be no dinar or dirham."

Good deeds such as prayer, zakat, and Hajj are not rejected because of debt.