If the bulbs are not comming on and you have replaced them already then its either a bad connection on the bulb holder or the control board is faulty.
What you need to do is remove the control module from inside the hob. Its the part that you use to select the speed and turn the light on. Open the plastic casing and remove the control board from there and see if theres any signs of burning on there. As sometimes when a bulb blows and if the hood isnt connected to 5 amp fuse in the plug then it can cause the control board to blow.
But if the board looks ok then remove the bulb holders and check the wires to the back of it and see if any have burnt or come out.
Check them and see what you find as this is will be either down to a bad connection on the holder or the control board at fault.