Is the key aftermarket?
Have you tried to jiggle the steering wheel back and forth while trying to turn the key?
Have you tried to hold the shifter in park while turning the key?
one more question...when you step on the brake pedal are you getting brake lights?
Welcome to Just Answer,
You very likely have a problem with the shift-interlock system. Lock cylinder failures are pretty rare nowadays. If you remove the shroud around the steering column you will find a cable with an an electrical connection.
Try this adjustment procedure:
If this does not work remove the cable from the column end and manually actuate the lever , you should be able to get the key to turn.
A retaining pin is located at side of key cylinder assembly.
Rotate key to RUN position.
Press in on retaining pin while pulling key cylinder from ignition switch
The ignition key must be in the key cylinder for cylinder installation.
Install the key cylinder into the housing using care to align the end of the key cylinder with the ignition switch.
I have never had a inop lock cylinder that did not involve the inter-lock, you will have to force the cylinder to get it to the run posistion, and this is going to cuse problems with the theft deterrent. You are probably going to end up at a dealer anyway to get the new key programmed.