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What is it doing? Does the blower motor blow air but it's not hot? Or do you get no noise or airflow when you turn it on?
Did you check the bottle to make sure it's full even though you may not have seen any leaks? It's very important that you check the bottle, it's very common for the coolant level to be low when you don't see a leak on the ground.
If the coolant is full the next thing you'll want to do is run the engine until it's full hot and feel the heater hoses at the firewall to see if they are hot. If there is a large difference in temperature between the hoses it's because the heater core is plugged and would need to be replaced or flushed.
If the hoses are hot at the firewall and you have no heat at all the next thing to do would be to check the operation of the blend door actuator. This is the door on the heater box that changes the temperature. The actuator or control head may have failed or the door itself could be broken.
If you need more help please hit the reply button rather than opening another question. I've closed the duplicate that was opened.
The blend door is inside the heater box, and the actuator is mounted to the outside of the box and drives the door. The blend door actuator is located direcly on top of the heater box next to the heater core, and requires instrument panel removal to get to the actuator.
Before digging into it make sure fuse 25 in the fuse box in the dash is good. if it is you could remove the a/c control head from the dash, find the dark green/yellow wire in the control head connector, start the engine and momentariily ground this wire a few times. This would drive the actuator and the temperature should change. If it does you know the problem is in the control head, but if it doesn't change and you do have good hot heater hoses the next step would be to remove the instrument panel and inspect the actuator and door where the actuator engages it.