When the pilot arrived at his planned destination, he was unable to obtain a down and locked indication for the nosewheel. Both normal and emergency means of lowering the gear were employed, but without success. A diversion was made to a more suitable aerodrome, where a safe landing was made with the nosegear retracted. It was determined that the hinge on the left door of the nosegear assembly had seized because of corrosion. On the previous retraction, the door linkages had been forced out of position in such a manner that the gear doors were jammed closed. All required maintenance and inspections had been certified as being completed, and it was not possible to determine how the hinge had become corroded.