Answer:
extreme traumatic stress- War neurosis- battle shock- for example- possibly civil angle such as massive trauma from witnessing horrific crime, fire or again, accident- which could involve head injuries- also such things as terrorism and political variations ( as the girl from Russia- who was always going blank when pressed on what happened in the fateful night in the Basement in Ekaterinberg! There are many causes.
There are many reasons to get amnesia, the primary reason is traumatic stress, some times old age can also lead to amnesia.