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I need to know the brand and model of your heater. If you don't know then some pictures with the access cover/panel off might help. You can post pictures here by clicking ADD FILES next to the SEND button
Without a model number all I can give you is general info. In most cases this type of unit has a thermodisk that senses when the heat exchanger is hot enough so the fan won't blow cold air. So you need to trace out the power leads until you find the controller disk. It sounds like the thermodisk has failed
That's trace the power leads to the fan
I can call you if you like but I don't see the benefit of a call since I don't have a model or pics (yet).
A well focused pic of the wiring diagram would be very useful