Hi Les,
You said that the coolent level was "up to normal" which sort of leads me to believe you were checking the level in the expansion tank. Sometimes you can have fluid in the expansion tank but the level is down inside the burner unit itself. You need to take off the radiator cap on the top of the Hydro Hot unit itself and check. It should be completely full, right up to the top.
You also need to check specifically what lights are showing on the control panel in the bay. A very common fault is that you will find the "low voltage" light illuminated. That can happen when the batteries get too low or sometimes if you have a momentary loss of DC voltage. Next to that light is a small hole in the panel that should be labled "reset". You can use a paperclip to push - gently - the reset button.
If one or the other of those doesn't solve the problem, read the troubleshooting section in the manual that Lee sent you the link to. It's pretty easy to use and you can always ask more questions here on the forum. Several of us have had cause to dive pretty deeply into Hydro Hot issues.