Dear Student,
The mitochondria (has the ability to self-replicate) that is worn out is digested by the lysosome, and new mitochondria is formed by the process called as mitochondrial biogenesis. If one mitochondria is not functioning, it does not means the cell is devoid of it. The other mitochondria present in the cell will compensate for the production of cellular energy until the reformation of new organelle.
Regards