A nervous breakdown has been called many things in the past. In the Middle Ages it was called melancholia. In the early 1900s, it was known as neurasthenia. From the 1930s to about 1970, it was known as a nervous breakdown.
A nervous breakdown is not a real mental disease, or condition. It really just means that a person has been forced to the point where they can no longer function in daily life. The closest true mental condition that can be associated with a nervous breakdown is extreme depression. They may also seem like panic attacks, schizophrenia, post-traumatic stress disorder or acute stress disorder.
Unfortunately, more than 1/3 of all Americans feel at some point they are on the verge of a nervous breakdown.
Since a mental breakdown is so closely related to depression, a good idea might be to treated as so, using medication and psychiatric counseling.
Sources
http://howstuffworks.lycos.com/question653.htm
http://my.webmd.com/content/dmk/dmk_article_1458605