The pilot of the amateur built gyrocopter reported that during taxi to the runway, he was distracted by his electronic altimeter when it entered a sleep mode. After resetting the altimeter, the pilot immediately entered the active runway and initiated the takeoff roll without disengaging the main rotor brake. Subsequently, at 25 miles per hour during the takeoff roll he reported an unusual vibration and a knocking sound. The pilot attempted to reduce speed, but the gyrocopter tipped forward and to the left, the main rotor impacted the ground, and the gyrocopter rolled over on its right side coming to a stop on the runway. During an interview with the pilot, he explained that the rotor brake must be disengaged before takeoff. He also stated that this accident may have been prevented if the rotor brake was designed in a manner that required manual hand pressure from the pilot. The pilot stated there were no mechanical malfunctions or failures with the gyrocopter that would have precluded normal operation. Substantial damage was found along the main rotor and vertical stabilizer.