AFTER ARRIVING AT THE DESTINATION THE LANDING GEAR COULD NOT BE EXTENDED. THE AIRPLANE WAS FLOWN BACK TO ITS BASE WHERE A GEAR UP LANDING WAS PERFORMED. EXAMINATION OF THE AIRPLANE REVEALED THAT THE LEFT GEAR DOOR DID NOT UNLOCK. THE EMERGENCY EXTENSION SYSTEM FAILED TO OPERATE AS A RESULT OF EITHER THE FAILURE OF THE CLUTCH ACTUATOR TO RESET AFTER THE LAST RETRACTION OR SLIPPAGE OF THE CLUTCH. THE PILOT'S REPORT STATED THAT THE DIRECTOR OF MAINTENANCE INDICATED THAT THE ACCIDENT COULD HAVE BEEN PREVENTED BY MORE FREQUENT LUBRICATION OF THE CLUTCH.