89 420SEL wouldn't start in "P"

[420]

So the other day I’m out running errands, and I hop in the 420 after completing an errand and the car wouldn’t do ANYTHING, nothing, nada, when I turned the key. I try several more times with no change. Then I decide to put gear selector in neutral and then turn the key in which case the car fired right up. This has only happened once. This was probably just a fluke but I was wondering what would cause the problem I had? What would need to be checked?

Thanks,