The student and flight instructor of the multi-engine airplane departed for a training flight and initially planned to practice touch-and-go landings in the airport traffic pattern. After performing a go-around during the initial landing attempt, the flight instructor described that the student configured the airplane for landing and verified that the landing gear were extended. About 2 seconds after the airplane touched down on the runway normally, the nose landing gear collapsed. The airplane came to rest on the runway and the lower forward fuselage was substantially damaged. Postaccident examination of the landing gear revealed that the lower bolt, which secured the nose landing gear down lock link to the lower drag link, was missing. The bolt was not found after the accident. Without this bolt, the nose landing gear could not remain in a locked position after it was extended. Given this information, it is likely that the bolt separated during the flight, which resulted in the nose landing gear collapse during the subsequent landing.