1997 Cadillac Seville

Computer problem
1997 Cadillac Seville V8 Front Wheel Drive Automatic

The scrolling message on the dash reads, "Theft System Problem" followed by "Car May Not Restart" The car does start however when the gear shift is moved from park down through nuetral and into drive positions the transmission does not engage. The trans does not engage in any of the positions -the engine idle fluctuates as you go from one position to the next. The trans will go into nuetral and was opperating normally prior to this -with no grinding, clunking or whirling sounds. Nor is it leaking or requiring added fluids frequently. What is wrong and how can I repair the problem.