Anti-theft system problem car may not be restarted?

The normal reason for that message is if you were driving and there was a failure in the anti-theft system. It warns you that if you shut the engine off it may not restart due to the failure. Your car is one of the last to use the Pass-Key VATS system with the resistor in the key. A common failure in them is if the two contacts that read the key get dirty or the key gets damaged, so it no longer makes a good connection. The next most common failure is if the two wires that carry that reading from the ignition cylinder to the VATS module in the instrument cluster break. Those are the circled part of the system. One of the first things is to see if the key is very loose in the cylinder, the cylinder may be worn enough that it doesn't make good contact. If the message only shows up intermittently or if it shows with one key but not another that is the likely reason. The repair for both failures is to have a locksmith or dealer order you a new key cylinder and key it to match the current keys and to make 2 new keys with the same resistor value as the current key(s) have. No real DIY repair method due to needing to key the replacement cylinder and then cut the correct keys. Replacing the cylinder on its own with a new one sometimes works but it still needs the proper keying to work. You cannot replace the parts with another used or new set say from a parts store or salvage because the resistor value needs to match the keys that you already have and there are 14 different values possible as shown in the table. The number 1 value was dropped in 1995 due to it being unreliable. It's a common failure and the reason why it was scrapped in favor of the pass-lock system of the later years. There is another method of "repair" but it actually disables the anti-theft system completely. It is easy to find written and video versions online if you search for GM VATS resistor bypass.