What would cause the heat not to work in 1998 Ford Contour Sport?

it could have a bad heater water control valve. the valve is controled by engine vacuum and the vacuum line to the water valve may have a leek or be broken In addition to the above it may have a thermostat stuck open, or a clogged heater core. Drain and flush the system, replace the thermostat, and install a 50/50 mix of anti-freeze and distilled water.