Could be a faulty thermostat or a problem with the ventilation fan (which isn't blowing hot air into the cabin). However, first things first, check the van's coolant/anti-freeze level.

Make sure you check the coolant when the engine is cold and the vehicle is parked on a flat surface. The liquid should be between the MIN and MAX marker on the water tank. If the fluid is low the system will not transfer heat from the engine to the heater core. As a result, you'll not get any heat from the blower.