What should a person do if he memorised some of the Qur’an then forgot it?
Question

If one forgets some part of the quran after memorizig it, and then repents, would one have to revise the forgotten part for the repentance to be valid? If it has to be revised, then the surahs or rukoos learned properly can be easily revised, but how can one revise ayat learned from random locations, when one does not remember those locations? Would this revision have to be done immediately, or can one take one's time in doing it (i.e. doing it in spare time)?
Praise be to Allah.Firstly: 
Undoubtedly studying the Quran, reciting it and memorising it are among the best of righteous deeds. The Prophet (blessings and peace of Allah be upon him) encouraged us to read Quran regularly lest we forget it, which means constantly reviewing what one has memorised and repeatedly reciting it. 
Similarly, forgetting Quran is a blameworthy action, because it involves falling short, turning away from the Book of Allah and abandoning it. 
See the answer to question no. 3704. 
Secondly: 
The scholars differed concerning the ruling on forgetting Quran: 
It was said that forgetting Quran is a major sin. 
It was said that it is an act of disobedience and sin, but it does not reach the level of major sins. 
It was said that it is a calamity that befalls a person and affects his religious commitment, or it may be a punishment from Allah for some of his deeds, even if it is not in and of itself a major sin or minor sin. This is the view that is most likely to be correct concerning this issue.
However it is not appropriate for the one who has memorised it to neglect his recitation or to be careless about reviewing it. Rather he should adopt the practice of reciting a daily portion in order to help him to remember it and prevent himself from forgetting it, in the hope of reward and of benefitting from its rulings. 
Thirdly: 
Forgetting some of the Quran is a result of neglecting it, and some kinds of neglect are less serious than others, as Ibn al-Qayyim (may Allah have mercy on him) said in al-Fawaa’id (p. 82). But forgetting it because of turning away from it and being distracted by other things is undoubtedly a calamity that may lead to further calamities, in addition to the loss of reward. 
What is advised for one who memorised some of the Quran then forgot it is the following: 
·He should revise the soorahs that he had memorised until he has memorised them properly once again.
·He should keep up with review in a regular fashion, so that he will not forget it again.
·He should carry on memorising and reviewing under a well-versed shaykh.
·He should review what he had memorised of large portions such as a juz’ or a hizb and the like, and strive hard to memorise an entire soorah, so that reviewing what he had previously memorised and has memorised again will help him to complete memorisation of the soorah.
·As for reviewing small portions that he had memorised then forgotten, such as two or three verses and the like, he should not focus on that and should not go to the trouble of remembering what he had forgotten of them. 
He should focus on re-memorising what he had forgotten of soorahs and large portions, as mentioned above, and there is no sin on him for not re-memorising those small portions, some of which he may have forgotten. He should examine his own situation, and whatever is a sin he should ask Allah to forgive him and repent from it; whatever is a shortcoming, he should amend it; and whatever was a case of turning away from matters of the hereafter and focusing on worldly matters, then he should refocus on the hereafter, for it is better and longer lasting. 
Moreover, it is better to put his efforts into reviewing what he has memorised of the Quran immediately, when he is still motivated and before he loses motivation and starts to procrastinate. Ibn al-Mubaarak narrated in az-Zuhd (1/469) that Ibn Mas‘ood (may Allah be pleased with him) said: These hearts sometimes have drive and focus and sometimes they slow down and become distracted, so make the most of the times when they have drive and focus, and let them be when they slow down and are distracted. 
Undoubtedly the fact that one who has forgotten Quran feels that he has fallen short and is asking how to restore it comes under the heading of having drive and focus after having slowed down and been distracted. One who is in this situation should start re-memorizing it immediately, and should not delay doing so. 
If he is not able to revise it except during his free time, because he is very busy, has a lot of work and is striving to earn a living for his dependents, and the like, then there is no blame on him. 
And Allah knows best.