One assumes this problem is with an electric cooker or hob, despite you posting in the gas cookers section.
The switch has probably failed. Normally these are easy enough to source and to replace however, please realise you will need to have the make and model of your cooker handy when you search for parts FOR YOURSELF. For some reason you didnt bother to mention either of these items in your question
Not a psychic, October 2016
