Please help! What is the cause of mitochondrial disease?

Genetic change. Mitochondrial diseases can be caused by changes in mitochondrial dna (mtdna), inherited only from mother but most are caused by changes in genes inherited both from mother and father (nuclear genes). It can be inherited in other ways too. More info on the complex inheritance of mitochondrial diseases: http://www.Umdf.Org/site/c.8qkoj0mvf7lug/b.7940911/k.555f/inheritance__genetics.Htm.