All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
If the pledge had the nature of a commitment, then it is considered a vow. It is obligatory upon the vow taker to pray Adh-Dhuha and perform voluntary night prayers in fulfillment of the vow because the Prophet, sallallaahu ‘alayhi wa sallam, said: “If one vows to obey Allaah Almighty, then he should obey Him.” [Malik, Al-Bukhari, Abu Daawood and At-Tirmithi]
If one takes such a vow and fails to fulfill it on some days, then it is obligatory upon him to make up for what he misses as long as he is able to do so. One is not absolved of the vow by not fulfilling it.
Therefore, it is obligatory upon the questioning sister to make up for the prayers that she vowed to offer but did not. She has to pray Adh-Dhuha every day and perform voluntary night prayer every night.
Concerning night prayers, if she did not intend a certain part of the night or a certain number of Rak‘ahs (units of prayer), then it suffices her to perform two Rak‘ahs at any time of the night, even at its beginning.
Allaah Knows best.