With remote locks, if they're working, a frozen lock cylinder won't keep you out of your car. This was the door being frozen.

I've solved this a few ways. The hatchback in my girlfriend's car was the only way in, so I crawled forward and started the car from there a couple of times. My current car has a remote starter, so as it warms up from the inside the door can get a lot easier to open. If I can get to the ice scraper (sometimes the trunk is easier to open) I can use it as a chisel.