Make your car fan auto. Heater has its own blower located just under the dash board in the middle of it. Turn hot/cool rotator on left side of the meter to Red i.e. hot and start the heater fan. It should turn on the heater, if its pipes are not detached by mechanic. If you still don't get hot air, there is a cable that is attached to rotator selectors. Check if this is attached on a knob in the heating assembly. It is quite possible that this cable is detached with foot while driving the car.