overheats in cold

We've had a cold snap here. The very first day my 88 caprice has no heat coming from heater and it overheats and boils over on the way to work. The only parts in this system are the water pump and the thermostat. Which one do you think is the problem? I have an 88 Caprice Classic, 305 w/air.

This happened to me once. You have too much water in your coolant, and ice formed (odds are in the lower radiator hose like mine did). The ice acts like a cork and doesnt let the coolant flow to the engine and heater core, and therefore overheats the engine and boils over.