Just before the loss of consciousness is a period of anterograde amnesia that results from an erasure of those memories laid down before the loss of consciousness. The duration of the anterograde amnesia is proportional to the duration of the loss of consciousness. Because of this amnesia, a fighter will probably not remember what occurred just before the knockout occurred.
Once consciousness is regained, is a period of retrograde amnesia that results from an inability of the brain to lay down permanent memories after a loss of consciousness. The duration of the retrograde amnesia is proportional to the duration of the loss of consciousness. Because of this amnesia, a fighter will probably not remember what occurred for a few minutes after coming to after a knockout.
Knockouts and blows to the head are not good for fighters. Boxers with more than 50 professional bouts often have obvious Alzheimer like symptoms as well as MRI and psychological test abnormalities. In his book, Brain Damage in Boxer: A Study of Prevalence of Traumatic Encephalopathy Among Ex-Professional Boxers (London, Pitman, 1969), A. H. Roberts reported that 15-40% of former professional boxers had some symptoms of brain injury. He found that the longer career and the more professional fights a fighter had, the greater the prevalence of brain injury.