triggered the theft SYS light to come on and the car won't start?

Hello,

Many of these GM vehicles you can reset the security system by following few steps.

Try this method:
Unlock the car using the key fob.
Insert the key into the outside door handle and turn it right and then left to lock and unlock.
Insert the ignition key and crank the engine over.
Hold the panic button down to disable the alarm.
Push unlock button and try starting it.
If that doesn't work disconnect the car battery by removing the negative cable end.
The pass lock sensor that controls the security system is built into the key tumbler or tumbler case which reads a chip inside the key.
This chip is generally unseen.

On older GM cars two small wires will break near the tumbler which will activate the system. In this case the tumbler must be repaired or replaced.

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please let me know of any questions.

Thank you.