Any computer user would have had an (unfortunate) experience where their computer crashed and all the information there was destroyed in a second. You may still be able to format it and use it without problems, but the data you had on the computer and any customisation you made would be lost. But what if this exact thing could happen to a human being?
There are many types of amnesia, with causes ranging from neurobiological (where trauma to the brain, a drug or some other pathology causes memory loss) to psychogenic (where there is no apparent biological cause for the amnesia). With psychogenic amnesia, one only experiences retrograde amnesia, where they cannot recall memories from the past. However, anterograde amnesia, where you cannot form new memories and keep forgetting what happened, is absent in psychogenic amnesia. Psychogenic amnesia is often caused by extreme stress or a traumatic event. One type of psychogenic amnesia is situation-specific amnesia, as seen in post-traumatic stress disorder (PTSD) that occurs after a severely stressful experience such as war, rape, child abuse or witnessing a brutal death. In this case, the patient tends to only lose memories regarding the event, as if the brain is trying to protect the person from the hurtful memories.
A more interesting and much rarer type of amnesia is global psychogenic amnesia, also known as a fugue state or dissociative fugue. Unlike situation-specific amnesia, patients in fugue states have absolutely no memory of their original identity and personality. Simply put, they (usually) retain all their functions such as speaking and social interactions, but their persona has been wiped out like a formatted computer. Fugue states often develop after severe stress and can happen to anyone. Similar to situation-specific amnesia, the brain blocks all memories of the past in an attempt to protect the person’s psyche. Due to the “deletion” of the previous persona, patients in fugue states often generate new identities and begin wandering (sometimes even travelling to another country) away from the place they lost their memories. This is most likely the brain attempting to leave the environment to avoid the stressor that caused the event.
Fugue states are often short-lived, lasting from days to months. However, very rarely they can last for years. Once out of a fugue state, the patient recovers all of their past memories but have no recollection of what happened during the fugue state. This creates a hole in their memory. For obvious reasons, this usually causes intense confusion and distress in the patient and treatment is often based around helping the person come to an understanding about the episode and cope with the stressor that caused it.