most of the time it depends upon what your mental disorder is...

example someone with depression would have depression symptoms, someone with schizophrenia would have schizophrenia symptoms..

a more general definition for those without mental disorders is....

another name for mental breakdown is a nervous breakdown which is short term (can last minutes to just a few days) stress induced anxiety, depression, sleep deprivation, ...in short everything you would see in a mental disorder except does not fit the diagnostic criteria of mental disorders for duration of having the symptoms long term (30 days to years)

in other words its a temporary condition that usually takes care of itself with things like good diet, exercise, limiting stress, getting enough sleep and sometimes short term medications and other short term treatment options.