Theft system reset

There is a thirty minute learn cycle that could solve your problem, if it is that it does not recognize the key. First I would try another key that has worked in the car before. If both of them are refused it is not likely to be the key. In that case it could be a bad VATS sensor or broken wiring.

The process is simple.
Insert the correct key, turn it all the way to start. The engine should turn over but not start.
Let the key return to the run position. The security light should be flashing. Wait until the light stops flashing. (Usually right around ten to eleven minutes. Now turn the key off.

Do not remove the key.

Repeat the process two more times for a total of thirty minutes/three cycles.
After the thirty minutes and three cycles are finished wait about thirty seconds and try to start the car. If the procedure was successful it should start and run.