There are a couple of different things that could cause the loss of heat. First make sure your radiator and overflow tank are full and at there specified levels. Run the engine till it reaches operating temperature(between 180 and 220 degrees). Feel the upper radiator hose and lover radiator hose, they should both be HOT, this lets you know your thermostat is working. If the top hose is hot and lower hose is cold. Replace the thermostat. If they are hot, check to see if both heater core hoses are hot (2 5/8 in black hoses heading into the firewall).