Check that the can cable is properly plugged, then check if there is a problem with your CPU or your ram.
Try replugging the CPU power cord.
You may wanna go to shop and check it by using a different power supply.
Burnt out windings/coils, burnt out sensors in the fan, wires could be disconnected or damaged, the fan blades can be stopped from spinning by dust or other anomalies, the bearings could wear out enough to stop the fan from spinning,
Check the condition of the fan and try it in another computer or fan connector.