While driving my 2024 Hyundai Tucson Hybrid at approximately 20-25 mph approaching a red light, a sudden and total loss of brake power-assist occurred. Upon applying the brakes, the pedal became instantly rigid and rock-hard. Despite pumping the pedal multiple times and applying heavy manual foot pressure, the vehicle failed to stop, resulting in a low-speed rear-end collision with a stationary car stopped at the traffic light. No dashboard warning lights or malfunction indicators illuminated during or immediately following the event. After the collision, the vehicle's ignition was turned off and then restarted. Following this power cycle, normal braking functionality temporarily returned. The low-speed impact displaced an ultrasonic sensor in the front bumper. This intermittent electronic brake failure poses a catastrophic safety hazard to pedestrians and occupants if it recurs at higher speeds