Could be several things, but a common cause in other cars is that the cable running from the heat control to the valve that controls the amount of hot coolant that flows to the heat exchanger in the car is either jammed or broken. If jammed the rotary control that regulates heat would likely not move. If broken the switch could rotate freely. Also a possibility that if the cable is stuck the switch could have broken away from it and would still rotate. Finally there's the possibility that all is fine but the cable has become disconnected from the knob and just needs to be reattached.
Does the heater fan work? If hot coolant is flowing through the heat exchanger but the broken fan can't move any air across it you won't get much heat in the car.
Since the fan works and the knob turns it may be that the cable (assuming there actually is a cable) has become disconnected either from the back of the knob or from the valve controlling the flow of coolant into the heater's heat exchanger.
First suggestion would be to check coolant level and top off as needed, if it is still not blowing warm, check for air in the system, give me a call if you need assistance, the last week and this coming week I have been at Toms desk, 505-2100 x411..
