When the pilot selected the gear up after take-off, the retraction cycle took longer than normal. The gear was selected down prior to the next landing and although the gear up light extinguished the gear motor did not operate and the gear down light did not illuminate. Attempts to lower the gear using the manual system were unsuccessful and the aircraft was diverted to a more suitable airfield. During the subsequent landing roll the main gear, which was only partially extended, collapsed. Although the pilot was experienced on the aircraft type and carried out a trouble check of the gear system, he did not reset the hydraulic pump circuit breaker, which had tripped. The circuit breaker had probably tripped during the retraction cycle after the previous take-off. The reason the manual extension did not operate could not be determined.