What is the ruling on prayers performed with an incomplete ablution due to forgetting to wash the hands and fingers out of ignorance, and what must the one who did so do?

The hands are washed twice in Wudu' (ablution): the first time up to the wrists at the beginning, which is a Sunnah (recommended act), and the second time up to the elbows, which is an obligation (Fard). The first washing does not suffice for the second due to the difference in intention and the obligation of sequential order. Therefore, whoever washes their hands first and does not rewash them with the forearms, their Wudu' is invalid, except according to some Hanafis who consider the first washing an obligation. The previous Wudu' might be validated to alleviate hardship. It is advisable to gain a deep understanding of religion and to ask questions before acting.