need to replace the ignition key cylinder

If the key and lock cylinder turns like it should then you have another problem. Dead battery, poor connection or ignition (electrical) switch maybe. Also it is a possible starter problem. Any lights on dash light up with key on? Horn blow, headlights work?

Possible ignition switch. Those have an ignition box with gears in it, if that's bad it will not turn the switch.

also could be broken rack or rod in column.