A common issue is a broken wire going to the end points (wire got strained, bent too much or a whole range of other issues).
Another possible problem is the fan polarity (red and black wires) – if they are incorrect, you will blow the fan in seconds (trust me it is basically immediately – I blew one of mine).

Using your multi-meter (a very handy tool; at the very least you want to check voltage, resistance and continuity on 3D printers – if you need to buy one):

• unplug the wires inside the control box (always on wires should be the thin wires directly at the power supply, not running to the fan inside the box).
• unplug the fan at the top, you should now be able to check continuity (the resistance with a beeper function) on the wires (check the black wire then the red wire).
  • The multi-meter should beep or indicate a very low resistance (maximum a few Ohms, anything higher is an issue). If it displays an open circuit (each meter show this differently), that wire is faulty and will need to be replaced.

When you plug the wires in, make sure the red is on the + terminal on the power supply, and the black on the CG/0V/GND (it will use one of these to indicate the black).

If you want to test the fan, a safe way I have been doing is to get a 9V battery, and place the correct wires on the correct terminal (if the fan is on – it works; even though it is a 12V fan, it should go on with 9V and 9V might not blow it immediately) – you might need to make or get an adapter if the fan wires have a connector on them.