how to reset the theomostat ?

Hi ive had a brilliant indesit iz70c thats worked brilliantly for the last 18months. Now all of a sudden it has stopped heating properly, firstly the cooler setting and now the high heat isnt working.
Please can you tell me how to reset the theomostat as ive been told that might be the problem. I have already cleared the filter and do this everytime i use it.

We have to use this all the time as my son has pollen allergies and we cannot dry the clothes outside.