Door locks becoming hard to operate occurs around the world in very cold weather. The humidity in the air causes icing, metal components contract and 'jam' mechanisms, plastic components go 'hard' and refuse to slide.
There are sprays available, heated key tools, and other gadgets aimed at helping or avoiding it happening, with varying degrees of success.
If possible park the vehicle out of the wind to keep the exposure to the cold to a minimum. Try operating the other locks on the vehicle. One lock may operate whilst another lock will not.