Specific cases are usually described as a "breakdown" only after a person becomes unable to function in day-to-day life due to mental illness. At that point the person's condition is advanced, and seeking professional aid is likely advisable.
Like the term “sanity,” the terms "nervous breakdown" and "mental breakdown" have no medical definition and are not used in a clinical sense. However, the medical or personal problems precipitating a sudden breakdown may well benefit from professional medical or psychological treatment.
A mental breakdown is not the same as a panic attack, though mental breakdowns can trigger panic.
Causes of breakdown might include:
chronic and unresolved grief
chronic insomnia and other sleep disorders
serious or chronic illness of a family member
death of a family member
a traumatic, violent, or near-death experience
deception by a loved one.
The sudden, acute onset of the following mental illnesses might be classified as breakdowns:
post-traumatic stress disorder
Nervous Breakdown Symptoms