Car heater does not blow heat through vents or defroster onl cold air.
Heater does not work on 2003 Nissan Altima
Check the antifreeze level first in the radiator and the recovery tank (when the engine is cold). If its low, add as neccessary and get to a service facility to check for leaks. If the antifreeze is full (at proper levels) it will still need to go to a service facility and check other cooling system problems that may be causing your no heat issue.
Possible blend door actuator or thermostat?