Hi Martin and thank you for using Just Answer.
Your unit has a safety thermostat built in. If you remove the base-mat cap and press the red button it will reset the thermostat. If on an off chance this does not work, there could be a chance that the heating element could be burnt out or there is an interruption somewhere. In order to find the problem will require a voltage meter. I'm going to switch this to Q&A so if you have anymore questions, please ask. I wish you the best.