top tip - pouring boiling water over the door lock and heating up your key with a lighter often does the trick. 
drizzle, 2C/35F
It wasn't the key tumblers that were frozen.
It had rained for most of the day, then snow, then a deep freeze. There was a dam of "ice plating" holding all the doors shut. I drive a mini-van (Ford Windstar) and I could manage the leverage to pry open the tailgate, after busting loose a half inch thick layer of the stuff, and crawl in from the rear. Even when I got to work my (NOT insubstantial) shoulder took several slams to get the driver door open from the inside.

*dreams of palm trees, the sound of warm surf, sea salt smell, sandy wind and naked people*