01 Jeep Grand Cherokee heater only blows freezing cold air?

Your Jeep is 10 years old, coolant wears out, it needs to be replaces every 2 - 3 years, otherwise it could freeze in the hoses thus not allow the coolant to flow though the heater core. This is likely if you just suddenly lost your heat when the temp dropped well below zero. Many car owner don't change their coolant often enough.