Does one who falls asleep during prayer—due to an illness that causes him to lose consciousness, and then he awakes—need to repeat his ablution and prayer, or does he complete his prayer?

Ablution is invalidated by deep sleep in which the sleeper is unaware of what is happening, and there is no difference whether the sleep is standing, sitting, or lying down. As for light sleep (drowsiness) in which one does not lose consciousness, it does not invalidate ablution. If sleep invalidates ablution, the person must perform ablution again and pray upon waking.