When a person gets amnesia it's most times caused by head injuries that mess with the brain and then causes sudden memory loss.
Usually amnesia will resolve itself within a few days or a week but sometimes it might take longer.
People who have amnesia may experience loss of recent memory, but it develops suddenly and usually does not worsen with time.
Dementia is a disease of the brain that is usually not curable once it begins and progresses and overtime dementia usually gets worse to the point where the person with the dementia cannot remember how to do anything or may not even know who there children or grandchildren are.
Amnesia is memory loss like dementia but the amnesia is usually curable and fixable whereas the dementia usually is not curable or fixable.