A state of mind wherein one’s entire personal identity is abandoned and no other reasonable explanation (substance abuse, medication, a medical condition) is available. Also known as psychogenic fugue. Some causes of fugues include: extreme stress or trauma, accidents, reactions to natural disasters or the results of some form of sexual abuse during their childhood. This issue affects about 2/1000 individuals in the United States.
Example: A woman suddenly goes missing and is found wandering the streets in the next town, unable to recall who she is or how she got there.
Here is a great resource on causes, symptoms and how to diagnose a fugue state: http://www.merck.com/mmhe/sec07/ch106/ch106d.html