This can be several issues causing this. The heater control may be faulty, you may have a low coolant issue, a clogged heater core, a faulty thermostat, or a clogged radiator. Have a reputable shop check and diagnose your no-heat siutation.
Check out this answer: http://repairpal.com/profiles/dan-o. This is a common problem in 2002 Highlanders as noted below. This happened to me too. This comment is from a technician on this site "I WORK FOR A TOYOTA DEALERSHIP AND THE PROBLEM IS IN THE TEMP CONTROL SWITCH, THEY DO GO BAD, THE SWITCH COST AROUND $600.00 YOU HAVE TO BUY THE WHOLE CONTROL PANEL. CANT BUY JUST THE SWITCH". I had mine tightened repeatedly and eventually had to pony up the bill for a total replacement for about $700.