Hitting your.head hard can do what damage?

Most likely none: Most of us have struck our heads hard at some time. In most cases no damage is done. Even if you have a concussion, there is a concept of cerebral reserve which states that the brain has a 'cushion' of brain cells so it can tolerate small amount of loss over time. If after hitting yoiu head yoiu lose consciousness, have difficulty with orienttion severe headache, see your doctor asap.