The heat does not work in my 2002 Ford Explorer. Why would this be?

You heater core may not be getting coolant in. Sometimes these valves get stuck or vacuum line falls off or is damaged ..just one possibility ..see how it's in line on heater hose? Will go full screen Try opening it by hand and see what happens ..yours may not look identical but will be similar, this is 2002 Explorer

another angle ..could be control unit failed, it works by diaphragm... or valve it's self

I have a 2003 explorer sport with no heat, changed valve with vacuum, thermostat ,actuator ,flushed heater core..still no heat decided to change water pump and Boom ,HEAT finally....Car never over heated and didn't leak any antifreez. This all happened after my radiator change...