'01 Blazer Overheating Heat Won't Work.

My sister has a 2001 Blazer. She says that the car is overheating and that her heater won't work. I assume it's the heater core, but I'm not too familiar with these type of problems. Anywhere else I should look? Does it sound like I'm right about the heater core or is it maybe the fan or radiator itself?
-Lou

First check for overheating from lose of coolant or other.
If low on coolant the heater will not work.
Have you checked the thermostat?
Is it losing coolant?
Is the coolant low?