Okay, so I just drove down the road and hit a big long stretch of slush and then pulled into my destination and the car wouldn’t go into park. I turned around and drove back to my house (a couple miles away) and pulled into the driveway (which has a slight incline) I put the car into park (a little tough to put in the position) and let off the brake and the car started rolling backward making a clicking noise. I figured this was because there was a slushy up in there somewhere preventing the park from being engaged. I drove down to the dealership and put it in park there and it worked fine. (so I didn’t leave it there) However, when I went to start it, it wouldn’t start in park and I had to put it in neutral to get it started. I drove home and now it’s in the driveway in park but still won’t start in the park position. Do I need to bring it back to the dealer? Did a wire or something get jarred and is preventing it from starting in park? Do you think it will resolve itself when it dries out?
5 minutes after I posted this, I went out to try it and it’s working! If it stops working again, I’ll update you. Note to self: Avoid slush when possible even though I like the sound of it hitting the wheel wells… 
Also, on a steep driveway, put the HANDBRAKE on before you let you foot of the brake and put the car in Park. That will save your transmission’s parking mechanism and your car won’t roll into uncoming traffic.
I believe what happened is that the slush froze up around your transmission gear selector. This prevented the transmission from going into park all the way. When this happens the neutral safety switch will not work in the park mode and you will hear a ratcheting noise if the vehicle rolls back. I suspect that the transmission will work normally when the slush (ice) melts. You probably surmised this scenario already but I just like to see it in words.