Amnesia

Amnesia(a loss of memory)

Loss of memory as a result of brain injury or deterioration, shock, fatigue, senility, drug use, alcoholism, anesthesia, illness, or neurotic reaction.

Amnesia may be anterograde (in which events following the causative trauma or disease are forgotten) or retrograde (in which events preceding the trauma or disease are forgotten). It can often be traced to a severe emotional shock, in which case personal memories (in effect, identity) rather than such abilities as language skills are affected. Such amnesia seems to represent an escape from disturbing memories and is thus an example of repression; these memories can generally be recovered through psychotherapy or after the amnesic state has ended. Amnesia may occasionally last for weeks, months, or even years, a condition known as fugue.

source:Britannica encyclopedia