Heater

With this problem, I can give you 2 possible solutions. Your thermostat is bad and needs replaced or your heater core has plugged and no coolant can flow through. Betting on the thermostat though. Do need to know if you have overheating issues or if you can hear heater blower fan running.

Replace your thermostat if you have not done so. Also check your engine's fan clutch to make sure it is operating properly.