How can I get my oven to work ?

No the timer must be ok because you said the top element for the grill works. If fan bake is not heating up it is probably the element which is circular and fits around the fan on the back wall inside the oven. Take cover off and check elements for any signs of damage.e.g. holes burnts in it, splits or cracks etc. If it looks ok, turn oven on for a few seconds on fan bake. Turn off and see if element is getting warm. If not you will need to replace it.

Lincoln Appliance NZ
November 2008