What happens to a person's frontal lobes when they have a breakdown from stress? When a person gets really stressed out and has a mental breakdown what happens to their frontal lobes?

I'm reminded of the clip from "What about Bob?" when Richard Dreyfuss has a breakdown when he learns that Bill Murray is marrying his sister, and then he ends up in the home basket weaving for a couple of months as an example of this...
Research would suggest that the type of "incessant" or "severe" stress required to precipitate a  "breakdown" would the higher cognitive functions performed within the frontal lobes.  As I am unsure what level of understanding you are requiring on this topic, I have included suitable quotes from a relevant study, which provides the answer to your question. 

I quote Amy F. T. Arnsten's study of the effects of stress on the frontal lobe:


  The prefrontal cortex (PFC)—the most evolved brain region—subserves
  our highest-order cognitive abilities. However, it is also the brain
  region that is most sensitive to the detrimental effects of stress
  exposure. Even quite mild acute uncontrollable stress can cause a
  rapid and dramatic loss of prefrontal cognitive abilities, and more
  prolonged stress exposure causes architectural changes in prefrontal
  dendrites. Recent research has begun to reveal the intracellular
  signalling pathways that mediate the effects of stress on the PFC.
  This research has provided clues as to why genetic or environmental
  insults that disinhibit stress signalling pathways can lead to
  symptoms of profound prefrontal cortical dysfunction in mental
  illness.


Within the conclusion:  


  The detrimental effects of stress on PFC networks are particularly
  problematic in the ‘information age’, when PFC-mediated cognitive
  abilities are increasingly needed for success. understanding the
  molecular mechanisms that alter PFC structure and impair PFC function
  during stress exposure will help to reveal how genetic and
  environmental insults can increase an individual’s susceptibility to
  PFC deficits. Identifying the molecular mechanisms that alter PFC
  function will provide the foundation for a new era in psychiatry, when
  we will understand how genetic changes affect intracellular
  signalling, neural development and neurophysiology in the circuits
  that underlie neuropsychiatric symptoms. It is hoped that this
  information will provide the framework for therapies aimed at
  rectifying molecular errors and ameliorating the symptoms of mental
  illness.


Stress signalling pathways that impair prefrontal cortex structure and function
Amy F. T. Arnsten
Nat Rev Neurosci. 2009 June; 10(6): 410–422.
doi:  10.1038/nrn2648