From
http://en.wikipedia.org/wiki/Nervous_breakdown

Although not a medical term, the phrase "nervous breakdown" is often used by laymen to describe a sudden and acute attack of mental illness e.g. clinical depression, anxiety disorder, etc. in a previously outwardly healthy person. Breakdowns are the result of chronic and unrelenting nervous strain, and not a sign of weakness. Like any machine, the human body will start to malfunction when put under too much stress. In common usage the term "nervous breakdown" does not have a clinical definition and no reputable doctor would use it in serious diagnosis, instead focusing on definite symptoms and underlying causes. One common diagnosis used in this case is brief reactive psychosis.

Some commentators claim that a nervous breakdown can actually be a good thing in the long run, because (a) it forces the person to take a proper time-out to rest and recuperate and (b) the patient will have to deal with the issues that caused the breakdown in order to recover fully.

Causes of breakdown include chronic and unresolved grief, unemployment, career changes and other work stress, serious or chronic illness in a family member, divorce, death of a family member, and other sudden major life changes.