What is the ruling on forgetting many parts of the Qur’an after memorizing them due to negligence in revision for a university student, and does this entail a diya (blood money) or something similar, and how can all that has been memorized be revised?

One must regularly review the Qur’an lest it escape (from memory). Some scholars have gone so far as to say that forgetting the Qur'an is among the major sins, citing weak hadiths as evidence. Despite the weakness of these hadiths, forgetting the Qur'an after memorizing it is considered an ingratitude for a great blessing and a reprehensible state.