Amnesia can be organic-for example, resulting from psychogenic (i. e., resulting from a head injury; a brain disease a head injury; emotional shock emotional shock; a brain disease emotional shock; a head injury

a head injury; emotional shock

Amnesia with a organic cause it's when a part of the brain has a lesion due to a direct damage caused by physical trauma like a head injury or a disease.
Psychogenic amnesia on the other hand, has no structural brain damage but  occurs due to psychological stress, like an emotional shock.

Bhope this : )