The engine immobilizer is only turned on when the vehicle is locked and the "anti-theft security system" is turned on. To turn this off, you just have to unlock your doors with your remote transmitter. If your security light is still flashing, it is still on, and you have a computer issue. If the security light is not flashing, and the engine doesn't turn over, then your ignition key may have been de-programmed. GM or a locksmith that is familiar with pass-key III can fix it. If that is not it, then it has to be a mechanical problem, which i believe your first place to look would be your distributor cap/rotor, and go from there.