You'll have to check the fan motor, it may have failed. You can try jumping power and ground to the motor and see if it works, just be careful! Also check to see if the motor is getting a power and ground signal to turn on.
It is normal for the fan to come on when the sensor is disconnected. If the control module doesn't know what the engine temperature is, it makes the safe assumption that the engine is hot, so it won't over heat. Is your engine overheating or running too hot?
Sounds like you're saying the engine overheated, blew the headgasket and the radiator, and had a valve job performed too? Now it is running too hot when stopped in traffic? Is the gauge reading normal when driving? Is it overheating now or just running hotter than normal?