Most late-model cars use electric motors (called actuators) to change the air temperature and direction of the airflow by opening and closing flaps in the heater box under the dash. If your battery has gone dead, or you've disconnected it to work on your vehicle and now you can't change the air temperature, you probably just need to perform a recalibration.
Find the procedure in a shop manual (at auto parts stores for about $20). But if you're still not getting any heat and you've checked both heater core hoses to make sure they're hot, you likely have a bum actuator. You can usually diagnose and fix the problem yourself in about an hour. A new actuator costs less than $100. All you need is a shop manual, a 1/4-in.-drive socket set and screwdrivers. Here's how.
Consult the repair manual to identify which actuators control the defrost-vent-floor “mode door” and the temperature-regulating “blend door.” Then peer under the dash and locate the suspect actuator on the heater box. Have a friend switch between modes or temperatures while you watch the actuator. If the actuator doesn't move, it's probably bad.
There's a chance the mode or blend door actuator may be good but the door or linkage is binding, so check it before you replace the actuator (photo above). If the door binds, take it to a pro for replacement. However, if it moves smoothly, buy a new actuator from the dealer or auto parts store. Install it and test the operation. In some cases, you'll have to perform a calibration.