Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- Other scholars held the view that the only option left for one who missed prayers for years is to repent, ask forgiveness of Allah, and do lots of good works; by doing so he can hope to receive Allah’s mercy.
Answering your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
You are to be commended for taking the steps to repent and become a better Muslim; you can rest assured that Islam is a religion of mercy and hope; so be cheerful and trust in Allah’s promise of forgiveness.
Allah says, “O My servants who have wronged against their souls! Do not despair of the mercy of Allah. For Allah forgives all sins; for He is indeed Forgiving, Compassionate.” (Az-Zumar 39:53)
“Whoever commits a sin or wrongs himself, then (repents and) seeks Allah’s forgiveness, will find Allah Forgiving, Compassionate.” (An-Nisa’ 4:110)
Furthermore, Allah says, “Save him who repents and believes, and does good works; those, Allah will change their misdeeds into good works. And Allah is Forgiving, Compassionate.” (Al-Furqan 25:70)
Now let me come to the second part of your question:
Prayer is the most important pillar of Islam. It is in fact the first religious duty prescribed on every single prophet from Adam to Muhammad, second only to testifying the Oneness of Allah. Allah says, “Salah, indeed, is a duty enjoined upon the faithful at the appointed times.” (An-Nisa’: 103). The Prophet (peace and blessings be upon him) further narrates in a hadith Qudsi: Allah said: “The covenant between Us and them is Salah; so whoever establishes it establishes religion; whoever undermines it undermines religion.”
It is therefore important that we never become slack in performing prayer; if we miss any salah for whatever reason, we must pray it immediately without further delay. The Prophet (peace and blessings be upon him) said, “Whoever oversleeps and misses his prayer or forgets to do it, let him pray as soon as he remembers it.”
In light of these and other evidence, the vast majority of scholars and imams are of the opinion that one must make up for all of the prayers one has missed in life, no matter how many they are.
So according to them, you should make up for all of these prayers. One of the best ways to do this—as has been suggested by one scholar—is to pray with each fard that you perform another fard in lieu of what you missed in the past. Thus, for instance, before or after praying Zuhr, pray another four rak`ahs of Zhuhr as qadha’, and pray another four rak`ahs of `Asr every time you pray `Asr; you should continue to do this until such time that you can be pretty sure that you have made up for all of the missed prayers.
However, the above view has been rejected by scholars such as Imam Ibn Taymiyah, Shawkani, and Ibn Hazm. They are of the view that a person who has deliberately missed his prayers can never make up for them. Therefore, the only option left for him is to repent, ask forgiveness of Allah, and do lots of good works; by doing so he can hope to receive Allah’s mercy.
Ibn Taymiyyah, while advancing this point of view, further states: “To insist that a person who has strayed away from Islam for a number of years and then returns to the fold of Islam must make qadha’ of all his missed prayers serves only as a deterrent against his repentance, and thus it amounts to limiting the infinite mercy of Allah.” He, therefore, dismisses this view and rules that it is sufficient for him to repent, make lots of istighfar (asking forgiveness) and good works.
Having said this, I must add that you must never be slack again in your prayers.
Excerpted, with slight modifications, from: www.islam.ca
Moreover, the eminent Muslim scholar, Dr. Su`ad Salih, professor of Fiqh at Al-Azhar University, states:
Brother in Islam, bear in mind that in Islam, takleef (accountability) depends on maturity and sound mind. The Prophet (peace and blessings be upon him) stated that three people are not accountable for their actions: 1) the child until he becomes mature, 2) the insane until he is of sound mind, and 3) the sleeping person until he wakes up.
Accordingly, scholars of Islam state that being sane and mature are conditions for accountability. There are some reasons that could suspend the person’s accountability such as forgetfulness and compulsion. Then, a sane and mature Muslim should carry out his duties — such as prayer — as long as there is nothing that hinders him from doing so. Prayer is one of the most important pillars of Islam.
As for the prayers you have missed, you should make up for them at their due times. You can pray each prayer twice: one for the current prayer and the other for making up for the missed one.
Allah Almighty knows best.