It's possible for someone to forget everything after an accident. It all depends on the severity of the accident and the person's individual brain chemistry. If the accident was severe enough, it could cause damage to the hippocampus, which is responsible for forming new memories. This damage can lead to amnesia, which is when a person is unable to form new memories or retrieve old ones. Even if the accident wasn't that severe, it could still disrupt the formation of new memories, causing temporary forgetting. So while it's not guaranteed that someone will forget everything after an accident, it is certainly possible.