Just like a car running out of fuel, the cell can't function properly and stops working. These chemicals inside the mitochondrion are not part of the blueprint held in the cell nucleus. Each mitochondrion has its own DNA inside, which is the blueprint for the chemicals inside the mitochondrion. The genetic defects which cause LHON are in the DNA held inside the mitochondria. The DNA inside the nucleus comes from two places.
Half is from the person's mother and half is from the person's father.
However, ALL of a person's mitochondria come from the mother. Leber's is in my family.