Check and make sure there is water in the radiator. The car uses the hot water from the radiator to make the hot air that comes out of your heater. If there is water in the radiator and you have no water leaks, check your temperature gauge to make sure the car is not running hot. If it is running hot and has plenty of water you could have a bad thermostat. If it is not running hot and has water you need to have your heater hoses and core checked. If there is no air coming out of the vents though it could be a vacuum line problem, It is very VERY important that you make sure the car has water and that it is not overheating. You can ruin the engine in the car if either of those things are happening.
The heat in your car might not be working because of a problem with the coolant level, leaking hose or spoilt thermostat. You should check the coolant in your car to make sure that it is at the right level. Then, you should have a mechanic check out your car to find the exact cause of the problem.