The Car doesn't read the key fob in cold weather.

I had this happen to me last night. I locked the keys in my car yesterday morning and found a ride to my apartment to grab the other set. The keys stayed in my car all day on a cold Minnesota day, and when I went to start the car with the cold key, the engine didn't turn over. I used the other key that was with me and it started right up! When I got home, I shut down the car and tried the previously cold key and it worked. My key is not a fob, just a standard key, but it also contains a security chip that sends a radio signal to the car. This is added security so that you aren't able to start the car by jamming something into the ignition and turning it. Cold temperatures must interfere somewhat. Try warming the fob/key up.