Causes of car heater failure include a blown fuse, low coolant level or a bad vacuum. When replacing a fuse, be sure to use to the correct amperage rating to avoid causing damage to wire components.
If air is coming out of vents but not warming up, check the coolant level. The first sign of having a low coolant level is losing heat in the winter.
If there is enough coolant and still no heat, take a look at the car's vacuum. Refer to the vehicle's user manual for a diagram of the vacuum. Use the diagram to inspect the different parts of the vacuum. If any lines are loose, brittle or sticky, replace them.