What to fo when someone steals keys ?

You can replace the ignition tumblers and keys but as you don’t have the old keys it can get expensive cause stuff has to be drilled out and then pieces removed and replaced with new and then it will still need to be reprogrammed so it will start and run. The locksmith can cut new keys and reprogram the system for the new keys so that way the old keys will no longer work. It is cheaper to go with just cut new keys and reprogramming.

Answer not posting

Ask a Chevy dealer if they can use the VIN to cut a new key, but at twenty-two years old, it's doubtful. You could replace the lock cylinder & key, but it would still have to be programmed into the car, and that would likely cost a lot more than two-hundred bucks. And you'd still have to use the old key, which you don't have, to unlock the doors.

You can just replace the lock cylinder but the new keys will have to be programmed to the vehicle to be able to start the engine. Your Monte Carlo has transponder keys that must be recognized by the security system computer. If the computer doesn't recognize the key it won't allow the engine to start and run. But shop around. A different locksmith may give you a better price. Hope that helps! Jim