If the temperature indicator comes on then it's very likely the element is at fault.
If it doesn't come on, then the thermostat is the likely cause.
I'm advising on the most common cause of the problem you have.
Isolate the power to the appliance. Disconnect it from the electrical supply
I am assuming the temperature light is on, so the most likely cause of the problem is a failure of the circular element in the oven.
The method of changing will depend on the element fitted at the time of manufacture.
There are 2 types, one is held from the front by screws, the other is held from the rear by nuts.
If the interior light is on the rear panel, take the light glass off first.
Take the rear panel inside the oven out.
This will reveal the element and the fan.
Take the screws holding the element in place out.
Bring the element and the wiring into the oven cavity.
(Be careful not to pull the wires off as you manouver the element into the cavity, if you do, you'll need to take the oven out, rear panel off to be able to retrieve the wires. Don't try the machine if you have lost a wire, retrieve it first).
If the element is not obviously blown, you need to test it with a multimeter to see if it's open circuit.
Swap the wiring on to the new element. (Don't lose the wires into the back or you'll need to take the appliance out and remove the panels to retrieve them).
Fit the element in place and replace the cover.
Turn the power back on and set the clock.
Try the oven again.
If there are no screws holding the element in place, take the cooker out and take the rear cover off.
You can now access the wiring and the nuts holding the element in place.
If the element tests ok, let me know.
Obviously, if you cannot do this work yourself, you'll need to call someone in to do it for you.
Hope this helps
Let me know how it goes or if you need more help