What happens to someone who forgets who they are?

well, they'd have to learn to either accept this "start" of their lives and try to come to terms with reality and right and wrong and friends and family, and trust and other things, or they could try to pursue their past, via photographs, phone #'s, people... But, it also largely depends on if they still have their abilities they had before this amnesia, like intelligence. They would probably still be similar to the person they were, just with a new awakening they never saw coming.

amnesia occurs after head trauma & coma. Or due to dementia / brain degeneration. Its not possible to wake up from a coma then find yourself wandering round a city not knowing who you are... that only happens in Hollywood movies! You never just wake up from a coma, its gradual process, memory comes back as the head injury heals. People with dementia forget who they are, but they also lose orientation to time and place. They also lose insight, so aren't aware they are doolally!

It can happen, through injury or emotional shock, that a person is taken back to the state of infancy; needing to learn to walk, talk, etc. If all memory of their past is gone, but they are otherwise normal, they have to start a new life. And sometimes the old memory never returns.