What was found
(CAN) UNABLE TO RETRACT LANDING GEAR AFTER TAKEOFF. ACFT COVERED WITH ICE AND SNOW AFTER BEING LEFT ON GATE OVERNIGHT. REQUIRED ABOUT AN HOUR TO DE-ICE ACFT PRIOR TO DEPARTURE. AFTER TAKEOFF GEAR LEVER WOULDN`T COME UP. WHEN GEAR STARTED TO RETRACT IT TRIGGERED A SHOCK ABSORBER FAIL MESSAGE. CALLED MOC TO TROUBLESHOOT AND UNABLE TO RESOLVE PROBLEM, RETURNED. EMERGENCY DECLARED WITH ATC. OVERWEIGHT LANDING, APPROXIMATELY LANDING WEIGHT 68.3. MAINTENANCE ADVISED. FOUND NLG OLEO LOW. NLG OLEO SERVICED IAW AMM 12-12-32-611-001-A. FUNCTION CHECK OF NORMAL EXTENSION AND RETRACTION OF LANDING GEAR DONE IAW AMM 32-31-00-720-002-A. NO FAULTS NOTED.