A CPU usually isn’t working because it is too hot, so the fan is spinning and it cannot get air through. The problem can also be that there is dust in the vents and blocking airflow. The three main causes of a CPU not working are: 1) physical obstruction–dust or other objects preventing air from flowing into the CPU; 2) bad RAM contacts–a wire or connection has come loose from inside the computer’s motherboard; 3) excess heat–a cooling fan failure typically causes a big spike in system temperature, which can damage components if left unchecked.