Agreed with Jarpickle. Detailed lights on working theory of thermostat: Any liquid-cooled car engine has a small device called the thermostat that sits between the engine and the radiator. The thermostat in most cars is about 2 inches (5 cm) in diameter. Its job is to block the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. Once the engine reaches its operating temperature (generally about 200 degrees F, 95 degrees C), the thermostat opens. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions.
Like the thermostat, the cooling fan has to be controlled so that it allows the engine to maintain a constant temperature. The fans are controlled with a thermostatic switch and they turn on when the temperature of the coolant goes above a set point. They turn back off when the temperature drops below that point.
You should also check your fan switch. is it working or not ??