Is one who made a covenant with Allah to pray 12 rak'ahs of Qiyam al-Layl daily, then stopped for a period, required to make up what he missed, or is repentance and returning to his previous practice sufficient? And what is the advice for him?

A person must fulfill their pledge. If the pledge was accompanied by an oath, then sometimes failing to fulfill it necessitates an expiation for the oath, but there is no make-up for missed prayers. However, if the pledge was not accompanied by an oath, then there is no expiation for it, and the person should strive to fulfill what they pledged to Allah in the future to the best of their ability.