Ringo,

It's possible that there are 2 heating elements in this heater so, if you
are handy, check the lower element. With the power cut off check that there
is continuity across the element. With the power on check for voltage to the
element.
If it's just a single element heater the problem may be in the
thermostat. With the power off spray the thermostat with switch cleaner and
sweep the temp up and down a few times then reset the temp to the correct
temp.
While doing these things check all connections to be sure that they are
tight. Do not shock yourself.

Dave M.