If your rad fan isn't working then that would likely cause it to over heat. (I'm going to assume you live in a warm climate)
The air in your engine bay is hot, the air outside isn't. You need to be moving air over your rad in order to disipate the heat, cool the liquid and get that cool liquid back to your engine. If air isn't flowing over the radiator, the heat being taking in your coolant is not being removed, it's just cycling back into the engine.
It could be your thermostat. If that isn't working it wont let the antifreeze flow. And it wont turn the fans on. Does it overheat when your driving, or just when your in stop and go traffic? I'd replace the thermostat.