A disorder in which the memory fails with no known organic cause. Can occur spontaneously for a few hours or days (known as global dissociative amnesia), or after a traumatic event (situation-specific dissociative amnesia). Dissociative amnesia is also known as psychogenic amnesia.

This happens because of severe trauma/stress and is quite interesting because the entire continuity of the memories that are normally in the person’s brain is warped into a new way of thinking because of an extreme inner conflict. The person simply cannot accept that something has happened, so they dissociate.

Example: A woman witnesses the death of her child in a horrific accident. The next morning she arises and begins fixing the child’s lunch.
This website goes over the disorder in more detail: http://www.minddisorders.com/Del-Fi/Dissociative-amnesia.html