Amnesia
Amnesia can be permanent, or it can be temporary. Damage to the [brain], or the use of certain [drug]s can cause amnesia. Some of these drugs are [sedative]s. Another well known cause for amnesia can be drinking too much [alcohol]. Those kinds of causes are called organic, because they can been directly seen. Other causes cannot directly be seen, they are called functional.  Rather they are [psychology|psychological] in their nature. People may want to shield off a traumatic event they witnessed, for example.