My Apollo Rainier hot tub's heater doesn't seem to work - I get a FH message on the display. Please help!!!

HL generally stands for high limit fault which means a sensor is opening the heater circuit due to too high a heat reading. Common causes for such a fault start with poor water flow due to dirty filters which in turn allow the element to overheat. If necessary, replace the filter elements to restore proper flow.If flow issues are not the problem, try looking at the heater itself however you should probably call out a professional at this point as most remaining issues to check are within the controls and the heater and if you are not familiar with working on these components you should avoid trying to repair this yourself.