Re: what exactly happens when someone has a nervous breakdown?

I looked your question up using a web search engine and came up with what
follows which is an accurate description of what a nervous breakdown is. 
Of importance is the point that it is not a clinical/psychiatric term and
can mean many things to many people.
What follows is a good answer from How Stuff Works. 



Hope it helps,
gabriel vargas md/phd


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 . "Nervous breakdown" is a term that the public uses to
characterize a range of mental illnesses, but generally it describes the
experience of "snapping" under immense pressure, mental collapse or mental
and physical exhaustion. 
"Nervous breakdown" is not a clinical term. There is no psychiatric
definition of a nervous breakdown, and it has nothing to do with nerves.
"Nervous breakdown" is an inexact and unscientific term that is no longer
used in psychiatry. Modern psychiatry is breaking the term "nervous
breakdown" into more precise diagnoses. 
The diagnosis that most closely resembles what the public calls a nervous
breakdown is major depression. Depressive episodes may be caused by genetic
and biological factors and are often triggered by social and environmental
circumstances. Depression is defined as the "loss of interest or pleasure
in nearly all activities" and "sustained fatigue without physical
exertion." Depression is characterized by a lack of energy and motivation
along with feelings of guilt or hopelessness. It is often brought on by
stressful situations, such as relationship difficulties, health problems,
the aftermath of an accident or the death of a loved one. 
The mental illness known as a "nervous breakdown" may also be something
like panic attacks, schizophrenia, post-traumatic stress disorder or acute
stress disorder. 
Surveys show that about one-third of Americans feel on the verge of a
nervous breakdown at some point. Studies estimate that 50-million Americans
suffer some form of mental illness in their lifetime. 
Depression is treated through medication and psychotherapy.

References:
Kaplan and Sadock Synopsis of Psychiatry 6th Edition Lippincott Williams
and Wilkins

How Stuff Works
http://www.howstuffworks.com/question653.htm