If the fan in the main oven is running and no heat the most likely cause is the circular element has gone open circuit (blown).
Isolate the power to the machine before starting.
From inside the main oven, remove the screws securing the back plate, then remove the backplate, this will expose the element. Look and see if you can see where it has blown.
Remove the screws securing the element and carefully work the element and the wiring through the small holes into the cavity. (The wiring won't come in too far so don't pull too hard and lose the wiring).
Use a mutlimeter (if it's not obviously blown) to test for continuity through the element. If no continuity you need a new element.
Swap the wiring from the old element on to the new one and refit everything (reverse of taking it apart).
If you lose a wire into the back you will need to remove the oven from the cabinet it's housed in, remove the rear panel (lower) to be able to put the wire back on the element.
Good luck with the repair.