If your water heater is not heating water, chances are the heater’s pilot flame is not igniting. You can verify this by looking into the [water droplet-shaped] viewing window on the front face of the heater.
If when water is flowing out of the shower head, you cannot hear the igniter clicking repeatedly to light the heating flame:
Step 1
Check to make sure the igniter switch is on. The switch is located on the bottom of the unit, (pictured above).
Step 2
Check to make sure the igniter D-batteries are in good condition. These batteries should be replaced annually.(pictured above)
If you can hear the igniter clicking but there is no flame lighting, Check to make sure there is a full bottle of propane connected to the heater and screwed up hand-tight. Some times unscrewing and re-screwing will ensure the threads are airtight.