2001 Impala No Start-Security light

I have a 2001, and every once in a while my car will not start and the security light is on. I bought the car from a cop and do not have the key with the chip in it. A guy at the dealership gave me some way of resetting it and it will work so the car will start. My husband put in a new stereo 2 days ago and the car has not started since and the security light is on. The guy at the dealership told hjim to replace the BCM, ignition switch key and everything. And then told him to turn the car to on and leave on an hour and it will reset the codes and be fine (my hubbie is a mechanic) We have done all of this and it still will not start. I need some help if any one knows what to do? I need my car and I am becoming stressed it won't start. We have done everything the dealership said and I don't want to have to tow it to the dealership and pay another 500 on top of the 400 in parts I have already bought!!!!!

sounds like it needs to be reprogrammed you might have to tow it