Some of the tankless water heaters have a reset button others use the On/Off switch to reset. Obviously gotta have propane - check gauge, ensure valve open. Must have water pressure greater than 45 psi. If not, try using the water tank vs city water. I have a water pressure regulator with gauge permanently installed and set to 60 psi for city water (if it can get that high).
My water heater did not work for a week. Not sure if it was altitude (7,000 feet) or CG water pressure.
2015 Jayco Seneca 36 FK
2016 Jeep Wrangler Unlimited tow
2010 Tiffin RED 36 QSA - gone
2011 Monaco Monarch class A - gone