why will my 2001 LeSabre not start?

What is your question?

I installed a stereo in my 2001 LeSabre,my car was running normal when they pulled into the garage. After we finished with the stereo we were going to start it to tune it and see how it sounded, my 01 lasabre wouldn't start. It turn over and sounds like its going to run but it won't engage. I have checked the fuses and put a battery charger on the battery and tested it and it's at 12.6 volts. Please any ideas/advice is appreciated. I don't know what's wrong if it's the car or the stereo? Can you help me? thanks