Won't start, flashing security light

1999 LeSabre, V6, 28,000 miles, automatic trans. No sounds, smells, or fluid leaks.

The car was running great. I did nothing out of the ordinary but now it won't start, not even attempt to turn over. Battery is new and the cable connections are clean. All the accessories are getting power and work, ie. Horn, lights, power locks, trunk release. The only thing different is the Security light is flashing. When the ignition is turned to accessory the following lights come on: Engine, temp, and security.

I've tried 2 different keys and neither will start the car. The codes were checked and a 0 came up.

Any suggestions?