Cadillac Repair: 98 El Dorado & problems w/after market radio, market radio, professional radio

Question
I just had an after market radio installed and now the car will not start.  Is it the anti theft device causing problems?

Answer
YES, SIR. WHO installed the radio ?.  There are various voltagea going into and out of the original radio such as 5 volts, 12 volts and 16 volts. If you accidentally hooked up the wrong wires you can blow the computer and or engage the ant-theft system. Have a professional radio installation facility test it for you.

There is no way for me to know what is wrong without seeing how it is wired.