Do you have a heat pump or Gas Furnace? With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!
Car Blows Cold Air.
Could be a clogged heater core, stuck thermostat, sensor or relay ( if ur car has electronics). Radiator may need flushing . Worse case, combination of noted possibilities. Good...
Not much to go on but... First, BE SURE YOUR VEHICLE'S COOLING SYSTEM IS FULL! If not, FILL IT! If it keeps 'disappearing' after a few days or a few weeks, your engines gaskets are probably worn...
Theres a couple of things that can cause this. First, check ALL the hoses to be sure you dont have a leak. make sure the radiator and reservoir are not leaking. Then you need to check the thermostat....