What should I do about my regression from maintaining prayer after I had been diligent in it in my youth? And how do I make up the Fajr prayer if I wake up after noon?

A Muslim must maintain prayer, as it is the second pillar of Islam. Scholars have overwhelmingly held that whoever abandons it is guilty of major disbelief (kufr akbar) that expels one from the faith. If someone sleeps through a prayer or forgets it, they must pray it immediately upon waking up or remembering it, due to the Prophet's (peace be upon him) saying: "If one of you sleeps through a prayer or forgets it, let him pray it when he remembers it." So, if you wake up at the time of Dhuhr, pray Fajr first, then pray Dhuhr after its time has begun. This is not considered combining prayers, as combining occurs between prayers that share a common time.