Not sure if you have an electric fireplace or a gas fireplace. But, from the way you worded the description of your problem, I'm going to assume that it's electric. I'm also assuming that all the other features of the fireplace work, but the heater doesn' t. If that is the case, below is what the Owners Manual recommends doing.
5. Flame effect operates, but heater does not blow warm
The TEMPERATURE button is a thermostat control for the heater. Each
preset temperature setting has a different thermostat setting as shown
to the right:
The heater will NOT activate if the room temperature is above the
thermostat setting. If the ambient room temperature is 78° F the
user will need to select Setting 3 in order for the heater to be activated.
It is normal in warmer climates and in rooms with higher ambient temperatures
for the heater to not activate on the lower settings due to the thermostat
5a. Try increasing the temperature setting. See drawing:
6. If unit heater does not function, but Power and Heater
switches are in the "ON" position and thermostat is set
6a. Turn all switches to the "OFF" position
and unplug the unit from the wall outlet for 5 minutes. After 5 minutes
plug the unit back into the wall outlet, and operate as normal.