I have a 1988 buick lesabre that has cold a/c but the heat doesn't work and I don't know why?

The heater recovers waste heat from the engine cooling system by diverting hot coolant from the engine radiator, to the heater under the dashboard. The heater control on the dash is connected to the diverter valve in the engine compartment by a push-pull cable, or perhaps a vacuum line. Check to see if any of these things have been disconnected or damaged. In some cases the heater under the dash has become choked with "mud" from internal engine corrosion by improper maintenance and must be flushed out. In such cases, the improper maintenance has likely been untreated water instead of water/antifreeze mixture in the engine cooling system... Good luck...