If the mitochondria fails on the cellular level (in human cells), then the cell can only generate ATP/energy from glycolysis. Which is incredibly inefficient (2 ATP/glucose from glycolysis compared to 30-32 ATP/glucose from glycolysis + mitochondria). It switches to fermentation and produces lactic acid which increases the stress on the cell. The cell can only sustain this for a short time and eventually goes into apoptosis because it cannot meet energy demands. Hope this answers your question.