If you are described as being "amnestic", this means you suffered some loss of memory.  It can also mean loss of ability to learn and recall new information.  Basically, its an impairment of the ability to remember things.  Head trauma can cause amnestic conditions.  It can be a by-product of concussions.  

Example:  An example you may see in a medical record would be if the doctor noted "Concussion - amnestic to event".  If this was a car accident, this would mean that the patient could not remember what happened when asked by the doctor.