Richmond Tankless won't turn on.

I have a Richmond Tankless water heater model RMTG-53DV for natural gas with a UMC-117 Main Remote Control. This morning I heard what sounded like a purge fan running but no hot water had been turned on so the unit was not firing. I checked it about a half hour later and the fan (or what ever it was) was still running so I turned the power off at the Main Remote Control. Now it won't turn back on again. If I hold the power button in it beeps after 3 seconds then show a "24" after 15 seconds. Will appreciate any thoughts on why It won't turn back on.
Thanks,
Jack

Please note: My tankless is a Richmond, not a Richland, I made a mistake in the name.