What was found
LANDING GEAR FAILED TO RETRACT UPON TAKE-OFF. MANUAL GEAR EXTENSION PROCEDURE WAS PERFORMED. MAINTENACE FOUND CIRCUIT BREAKER CB43, LOCATED IN THE MAIN POWER DISTRIBUTION PANEL INADVERTENTLY PULLED AND COLLARED FOR AN MEL "M" PROCEDURE THAT WAS ACCOMPLISHED FOR ELECTRIC HEAT DEFERRAL WHICH CONTROLS THE LANDING GEAR MOTOR. THE CIRCUIT BREAKER WAS RESET. LANDING GEAR SYSTEM OPERATION CHECKS WERE GOOD AND A SUCCESSFUL OCF WAS PERFORMED. AIRCRAFT WAS RETURNED TO SERVICE.