Do you mean the radio? I remember they often put codes in certain locations, google it. it's in the glove box, door jam, trunk, under hood or something. If it's the ignition, does it have a aftermarket alarm? usually a toggler switch for it is mounted near the interior fuse panel and you hold it up for like 5 seconds and push it back down to enable ignition
A dead battery needs to be charged. There's. no "code to turn it on." You can buy a battery charger at an auto parts store, or you could jump-start the car and drive it for awhile to charge it. If the battery won't hold a charge then it's time for a new battery.