When the pilot selected the landing gear down he found that the gear was not indicating down. He could see that the main gears were extended and the control tower confirmed that the nose gear was not extended. The pilot diverted to a clear area and gained altitude to conduct further attempts to lower the landing gear. All attempts were unsuccessful. A landing was later conducted on the main gears, with the pilot shutting down the engine during the landing roll prior to the aircraft settling on its nose. Examination of the nose landing gear found that a pin had fallen from a shaft associated with the uplock mechanism. As a result, the nose gear was locked up, with no way to release the lock. The pin was recovered when the aircraft was lifted. The reason for the pin becoming loose has not been established. It is located in an area which can not be checked during a daily inspection. The aircraft had flown about 65 hours since the previous periodic inspection.