I am assuming you mean no air is coming out of the vents?? Or is it just not hot ??

 

If you have no air coming from the vent (blower inop) and the rear defroster went out at the same time then I think you are having an electrical problem, maybe even just a blown fuse.

 

Check the all the fuses and make sure they are all good . If all the fuses are good then your mechanic will have to check the problem from there. You could have a bad blower motor, blower resistor or switch for the blower.

 

If the blower is working and its just not hot then that is another type of problem. This is usually caused by a blockage in the cooling system or a plugged heater core.