No engine light no communication no crank no start

My car has a broken gear second and forth. I been driving that with a broken gear for a while so now I bought the gear from scrap of Lumina SS 2005. I took that gear went to mechanic gave him gear to change it I parked my car on machine he disconnected the battery he lifted it up changed the gear connected the battery try to start the car and the car did not start. It is cranking, but not starting. After that he connected computer it is showing no communication and no engine light at ignition on switch. I went to electrician he said my fuses are all working one said I have to change the whole computer with ignition. I changed it still did not work, but before it was showing security lock sign on but after changing now its off. Call the key guy to reprogram he said no security issue. I have checked relay also every thing is good and I was sitting in front of car while he was changing. Mechanic just changed gear and touched nothing else. What could be the problem? Please help!