"Memories and knowledge..."

I wonder,... when a person loses entirely his memory.. after an accident let's suppose.. and has forgot how to speak, how to walk, how to eat, his parents, friends, etc,.. all his knowledge about everything... and has to restart his life like a baby again... where did that person go.??

Where did that person that was in that body go??... and who is the new person in that body?...

What happens when the old "entity" comes back? do they fuse together or do they fight?...

This scenario eludes me since quite a while... relates to this post concerning AI though..

Do you have any theory(ies)/hypothesis?... I'd love to discuss this subject :)