The emergency extension system on a 421C blows high pressure nitrogen into the down ports of the undercarriage actuators. I would suggest it didn't retract due to 2000psi of nitrogen remaining in the down side of the hydraulic system preventing retraction... Are you the pilot of this machine? Did you read the Pilots Operating Handbook? The aircraft should not have been flown following this incident and will need to be jacked and retractions carried out to reset the shuttle valve and also have the nitrogen bottle recharged..