Car wouldn't start in cold weather

Gas line freeze-up is almost unheard of, especially if you're using gasohol which has alcohol in it. That's essentially one gallon of fuel system antifreeze for every ten gallons of gas.

As for "what to do?", Do you want to try to fix it yourself or let a mechanic figure it out? If you tackle it yourself, the basic first step is to determine if you're missing spark, fuel pressure, or most commonly, both. If you don't want to diagnose it yourself, allow the same shop to look at it. They have already started the diagnosis. If you run to a different shop, they have to start all over. If the problem is related to a sensor failing at a low temperature, that may have not been apparent once the car warmed up in the shop. Often problems like this require multiple trips to the shop or the need to leave it there overnight. The mechanic needs to work on the car under the conditions that cause the no-start.