Could be condensation in the fuel lines. I had the same problem once last year. I normally park in the garage. Went snowboarding over the weekend and stayed at a cabin by the resort. A couple of days later when trying to go home in the early morning the car wouldn't start. It had been well below zero over those nights. I bottle of HEET and it started up right away.
There are two different colored bottles of HEET. One is less damaging to seals and such than the other. I don't remember which, but I'd go with the easier stuff. Just keep a bottle in your car and put it into the tank if/when the car doesn't crank after sitting in the cold.