While operating under active Full Self-Driving (Supervised) mode the Tesla abruptly and without any driver input shifted from Drive into Neutral while accelerating through a curved highway on-ramp at highway speeds. This uncommanded gear change caused an immediate and total loss of motive power, resulting in the vehicle colliding with the guardrail before I was able to manually re-engage Drive. If I had not reacted promptly and calmly all passengers could have been killed with a collision at high speeds. The earliest available appointment for inspection at a Tesla service center has been scheduled for December 23. There were no warnings at all.