I have a Schlott Ceran Ariston Cooktop. When I press the On button the light indicators show at zero. When I first used the cooktop I would then set the temperature by pressing the - and + buttons which made the 0 change to the appropriate temperature. Now, however, when I press the - or + button, a LL appears and is flashing.

What does this mean? How do I fix it so that I can use the cooktop?