Will not start after installing new battery

I changed the battery after it sat up a month more while being repaired at a body shop. one of the repairs was a new door on the drivers side so my key will not work the cylinder. From what I've read maybe it is the anti theft system or immobilizer. I've tried resetting the security system by turning the key to the on position and leaving it there some length of time with no luck. another suggestion was to use the key in the drivers side door but that is not an option as my key doesn't work in the replacement door.