Retrograde amnesia is often experienced after a person is struck hard on the head, affecting the temporal, parietal or frontal lobes, which are the areas of the human brain where memory is stored or created. Typically, this form of amnesia is the condition of being unable to remember any experiences or knowledge before a severe head trauma or the onset of mental disease/disorder. Most often, amnesia of any form lasts only a few seconds to a few minutes, but many cases lasting as long as days, weeks or months have been recorded. The general consensus is that the duration of memory loss is somewhat proportional to the extent of damage or trauma- the harder you get hit, the longer it takes your brain to reconfigure itself.
Imagine your brain as being a very special kind of vinyl record player. Somehow, the needle gets bumped and is knocked out of the groove. You're no longer listening to the second track of the album, but track 10 is now playing. Now, this special sort of record player is made in such a way that it can adjust itself and force the needle arm back to the position it was in before getting bumped. It automatically begins searching for that particular groove in the vinyl, tracing back from the current position of the needle. Depending on how far away the current position of that needle is from where it was is going to affect how long it takes to find the previous position. Just a couple of grooves away from within track 2 will be relatively no problem, but tracing back from track 10 to track 2 might be more difficult.
Amnesiacs often experience mood swings, different behavior patterns, disorientation, depression and other psychological phenomenon that is considered "abnormal behavior." If you are encountered by a close friend who is experiencing retrograde amnesia, it is best to not force them to remember anything and instead be supportive and sensitive to their condition. If they ask questions, go to great lengths, if possible, to be completely honest. Try to keep in mind that their condition is temporary- eventually, their memory will return.
Amnesiacs who end up recovering often remember their experiences had while in their amnesiatic state. Actually, they almost always do. Some people think it might be fun to "toy" with an amnesiac's fragile state, but in the long run it can cause more harm with devastating effects.
Personal note: If anyone here on E2 has experienced this form of amnesia, I would be very interested in hearing about your ordeal. I want to write a fictional story centered on this form of amnesia, from the patient's perspective, but I am lacking actual information from real people who've had amnesia and want to make it "real." I'd go buy a book, but I'm chronically broke.