Paul,
Is it a combined heater and shower unit or a heater which then feeds the shower? If it is the latter, my guess is that the mixing valve has developed a problem. The water heater is heating water, but the cold water is overpowering any hot water coming thru the mixing valve.
If you let the shower run, does the water heater thermostat cycle on and off? If so, the heater is functioning, just that no water is leaving the heater.
Larry C
