There are 2 types of organic amnesia--retrograde, which is what Goldie Hawn's character in Overboard experiences, and anterograde, which is what Drew Barrymore's character in 50 First Dates has.
Myth: Getting knocked on the head will make you lose your memory.
You can't believe everything you see on television. You can't lose your memory forever just by getting a nasty hit to the head. Organic amnesia is a lot more complicated than fake doctors would have you believe. Watch this video if you'd like to learn what actually causes permanent memory loss.