If the amnesia is caused by an injury or other less severe brain damage then you usually forget things such as where you are or what's happened recently or who people are.
Amnesia due to an injury such as a fall etc is usually temporary and it after a day or so the amnesia will usually go away and if not then you probably have more severe brain damage and more severe form of amnesia.
I had amnesia before due to falling on my head and the amnesia made me forget who my friends were, who my mom was, where I was at, who I was etc but after a week the amnesia went away and I begin to remember things again.
If someone has amnesia they should see a doctor or go to the emergency room to make sure it's not a worse case of amnesia.
Usually with some medical treatment and rest the memory should slowly come back as long as you don't have severe brain damage.