The inability to access memories formed, or difficulty in retrieving them, before brain damage or injury is called a Retrograde Amnesia
It is caused by impairment to the memory-storage areas of the brain. This type of damage can result from a distressing injury, a serious illness, a seizure or stroke, or a degenerative brain infection. It can be temporary, permanent, or get worse over time depending on the cause. With this type of amnesia, memory loss typically involves facts instead of skills. For instance, a person might forget if he owns a car, what make and model it is, or when he bought it, but he will still know how to drive.