If you neglect to write something down, photograph it, or otherwise document that it happened, it's unrecorded. Many unrecorded events in history are passed on through oral storytelling.
When an event is recorded, it's filmed, described in writing, or somehow documented so that it can be referred to later. Anything unrecorded hasn't been captured in a permanent form. If your cousin's violin recital is unrecorded, that means no one took a video. The stories of people who hold little power in society have often gone unrecorded, so they're frequently left out of history books. The Latin root of unrecorded is recordari, "remember."