What would cause white smoke to come out of a muffler if a car was not run for a few weeks but was fine prior?

White smoke out of the exhaust is a sign that the head gasket is bad and is allowing water (coolant) into one or more of the cylinders. If the smoke decreases when the engine warms up, this may mean that the gasket is expanding when the engine warms up. Check for coolant in the oil by looking at the oil on the dipstick. A sure way to tell is to tell is that the oil will be a milky color and overfull.