What is the ruling if I did not pray Fajr, believing that its time had ended?

If the Fajr prayer is missed due to sleep or forgetfulness, it must be made up immediately, based on the Prophet's (peace be upon him) saying: "Whoever forgets a prayer, let him pray it when he remembers it; there is no expiation for it except that." There is no sin in sleeping before the prayer time begins. However, one should take measures to wake up, such as setting an alarm, otherwise, negligence would result in sin. But if the prayer is missed due to carelessness or deliberate abandonment, the matter is more serious, and repentance and prompt make-up are obligatory.