Hello, I just repaired exactly this type of unit last weekend. The problem was that the fan wasn't working at all. The fault turned out to be the switch - but I only found this out after removing the coker hood and everything unit from the wall - which wasn't actually necessary - but I only found that AFTER I had done it.
If the unit makes a different kind of humming that increases in pitch with the pressing of the speed control switches, this suggests that the motor is turning - i.e. still OK. If the fan was jammed, it would simply hum at a constant rate, quieter for the low speed and louder at the high speed where more power is being fed to the motor. I suggest that the nut that holds the fan blade to the motor has come loose, allowing the motor to spin but the fan won't turn - or not very quickly anyway.
To repair this, FIRST TURN OFF THE POWER TO THE COOKER HOOD, then the fan motor/assembly needs to be removed from the hood. The square stainless steel cowling "chimney" above the main part of the hood is NOT screwed in place (it is a decorative cover) and can be removed by hand. The back edges of this cover clip around the "chimney" part above. Remove this cover and you will then see the top of the fan/motor and 4 screws which hold the motor in place. Undo the 2 screws holding the flexible exhaust pipe to the fan. Remove the grease filters under the hood, then undo the screws holding the fan in place. Support the weight of the fan when undoing the last screw. Remove the fan. Undo the 5? screws holding the plastic cover over the fan. Check the brass collar nut which clamps the fan to the motor shaft. Loosen it, then tighten firmly. Check that the fan turns freely. Suggest you drop a couple of drips of "3in1" oil into the motor bearings whilst you have the access. Replace the fan covers and switch on fan. Other info - to check that the fan motor is OK, you can check the resistance between the fan motor windings. To do this you will need to remove the switch board from the cooker hood - 2 screws from the rear. The switch circuit board has 4 wires, marked C, 1, 2, 3 on the circuit board. The resistance is measured between C and each of the 1, 2, 3, and should be around 33, 45 and 52 ohms. Good luck with it!!! Ross.