How Do People Wake Up From Amnesia?

Amnesia can be caused by a variety of things. As can coma. People don't usually just 'wake up' from coma, it usually happens gradually and they go through stages of recovery.
Amnesia means loss of memory, and in real life it is not like in films where people apparently forget everything, but carry on living a normal life having forgotten all their past. In real life people with amnesia have forgotten a lot of their past but not everything, sometimes they can't remember what happened 5 minutes ago but remember their childhood very well.
People recovering from coma are usually very confused, very frightened (nothing makes sense to them) and because of that they can be very aggressive or very depressed and quiet. Everybody reacts in different ways, so anything you write about someone coming out of a coma could possibly be true, so don't worry too much about getting your facts right! One thing which rarely happens is the thing you most often hear about, and that is people waking up and being just fine and dandy.
Good luck with your story!

I think you wake up naturally, but I am not sure.

What do doctors ask somebody after they have woken up from a head injury?  How do they test for amnesia? What sort of questions do they ask?