Is alarm system connect to the computer system?

Hello,

Yes, if you have the anti-theft/security system triggered the vehicle will not start. It is a safely feature.
You will have to reset the security system before the engine will start.

Here is a general reset procedure for GM vehicles:

First unlock the car using the key fob.
Then put the key into the door lock cylinder and turn it right and then left to lock and unlock.
Get in the car, put the key in the ignition and crank the engine.
Hold down the panic button (it should disable the alarm)
Then press the unlock button.
Try start the engine.
If you are unable to start it, disconnect the negative battery cable.

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

Please let me know of any questions.

Thank you.