Could you tell me what happens if depression is left untreated?

Various outcomes: It may become chronic, get worse or evenutally resolve. If someone has depression, it is best to engage in aggressive treatment. This leads to the best possible outcome.

Depression: Difficult to predice, sometimes depression lifts on its own, other times it becomes worse. It is not worth taking the chance of depression worsening, see a mental health professional who specializes in treating depression. Best.