I unhooked my c.d player so I could plug the radio wire in the back of stereo..while I had the car in on position but not completely on..as I was putting the stereo back in dash board my car shut down
That's because you had the key in the on position and forgot to remove the negative battery cable when doing any wiring work on the car. I suspect you will need to check the fuses to see which one was blown from the wiring short that you caused.
Search Buick LeSabre Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Buick LeSabre Experts