This can be caused by two reasons
There is a safety switch on the heating element which ensures that the heating element does not become hot if it is not properly mounted on the housing/outer pan. You must ensure that the heating element slides exactly with the blocks into the guide of the outer pan so that the safety switch is pressed and activated. Only then will the heating element become hot again.
It is possible that the fryer has become to hot and as a result of that the overheat protection has started.
You can 'reset' the fryer to undo this. The reset (restart) button can be found on the rear of the heating element. You can activate the resetbutton by puncturing the hole with the attached rod or another thin object (such as a toothpick) and pressing on it.