1996 CADILLAC ELDORADO

I have a crank no start.......I changed out everything all the way down to the fuel pressure regulator.....Im not getting spark....one minute im driving,next minute im stranded.....one day after changing everything out the car started....I drove it maybe 5min then it died.....Im thinking maybe the security system locked me out.......no security light flashing.......when i go to put my key in the driver side door the alarm sounds off.......so i have to go to passenger side & put key in & turn it to stop the alarm......my key fob battery is not working @ this time.....everything on the car works and i do mean everything.....the owner previous to me had a aftermarket radio installed & put the stock radio back in before he sold it to me this could have something to do with it because when i bought the car i drove it home 45 min drive & the car ran smooth as a babys back side......got car home & then this problem came about......PLZ can you help me!!!!!!!If I have to get my PCM reprogrammed i will do that.....if i have to get a new PCM i will do that......just plz give me your professional input on this situation.......