Spark and/or Fuel/air mixture problem

Spark: Check for spark like Scott said, if it's there, have someone check the gaps on the spark plugs. Make sure they're clean. Are they pitted?

Fuel/Air or Choke: When temps get colder, the car's computer will try to reduce the air going into the combustion chamber. This normally wouldn't be a problem on its own (it makes it start easier on cold days), but can aggravate another problem. Try pressing harder on the accelerator, this should force the air intake to open up and get more air.