2000 Cavalier catalytic converter was stolen/check engine light

The check engine light is probably indicating a catalytic converter malfunction the Oxygen sensor that is located after the cat tells the computer if the cat is functioning properly it is possible the wire has been broken or damaged. To see if the cat is there jack up the car and look under the passenger and driver area of the car follow the exhaust pipe from the front back the first thing the pipe comes to should be the cat it looks like a muffler with a wire going to a sensor after toward the back side of it, I doubt they never installed one but I guess it is possible.

Crawl under the car and look for the catalytic converter. It should be mounted just after the exhaust header before the exhaust pipe. It looks kind of like an over-inflated football. If you don't see that, go back to the shop and demand they do it right this time (for free) or else. If you do see the converter, take your car back to the shop and explain that the check engine light is still on.

Yes, you should be concerned that the check engine light is on.

First of all, if your cat converter was replaced, they may not have installed the correct pipe where the downstream O2 sensor mounts, which is at the back of the cat. It measures cat efficiency. If that O2 sensor is missing, you can replace that cat a hundred times, and the check engine light will continue to come on after it's been reset. I suspect that the cat thieves may have cut the wiring that runs to the O2 sensor, and this was overlooked by the muffler shop, who usually don't work on or aren't skilled in OBD I and OBD II engine/emission control systems.

Look under the vehicle at the rear of the cat, and look for the O2 sensor, which resembles a spark plug with wires coming out of it. If that isn't there, look to see if there is a whole where the O2 sensor should mount in the pipe at the rear of the cat. I have a strong feeling there isn't one there and the muffler shop used stop tubing and their mandrel bending machine, to make up a new pipe and used a universal mount cat.