First post so excuse my ignorance : )
Here's my setup.......
Navien CH-240, remote controller, outdoor temp sensor, external thermostat on main floor of home, cast iron radiators

I had the Navien installed in late December last year and it ran fine until just yesterday. Now, no heat though I do have not water. Yesterday a contractor turned the heat off to work on a bathroom radiator. When I got home I noticed the unit was back on but not producing heat. When I looked at it there was an error code indicating low input water pressure. I noticed the input was shut. I turned the Navien off then slowly turned the valve on; it did have to fill a bit. Turned the Navien on, no more error code. After that I did not check it again, nor did I make sure the heat was working as it was pretty warm here yesterday. This morning I turned the thermostat up a bit as it had cooled off. I heard the circulation pump come on. After a little while I noticed it was not getting warmer. I checked, the Navien was not running; only the circulation pump was running. I have reset the Navien several times; I turned the remote controller off/on, the Navien itself off/on and pulled the power plug out of the wall - while it was off of course. I also turned the thermostat off/on. Still no heat. Is this thing working normally?? You'd think that when you turn the thermostat up you get heat. Is the outdoor temp sensor somehow overriding the thermostat? Thx for any replies.