1996 Lexus Es 300 Changed Rear After Cat Convertor Oyxgen Sensor Couldnt Get

I changed the rear after the cat. convertor oyxgen sensor . I couldnt get the car started. so i put the old one back on. still can't get the car started. I disconnected the negative terminal to clear the codes. I had the car running to heat the exhaust in hopes of trying to loosen the nut easier. when the nut broke loose the car stopped running. didn't think this was a problem so i swithed out the sensor. i put the old one back on. still cant get the car started.