1987 Ford Ranger ignition key cylinder

I have a 1987 ford ranger that I just purchased for my son. The other day when he went to start the truck he put the ignition key in and tried to start it. The ignition key did not stop it made a complete circle and the dash lights were on as if you were about to start the truck. I bought a new ignition cylinder (I think that is what you call it, it is where you put the key in to start the vehicle on the steering column) at a local auto parts store. I followed the directions and replaced the cylinder but I still have the same problem. What should I do next?