Verify that your shifter is securely in park. If the ignition cylinder does not see the shifter in park entirely for some odd reason, it will not allow the key cylinder to turn.
Also, make sure the steering wheel is not resting tightly against the steering lock. If it is, jiggle the steering wheel while trying to turn the key.
If none of that works, the key may be worn out (try another key?) or the cylinder may have a tumbler issue internally.