Why would a 50-gallon propane water heater just stop?

We don't know how old but it looks perfectly white and it is labeled as a
50 gallon water heater with clean readable labels.

Here is what we know.
1. The water heater has been working for more than 5 years.
2. Last night and this morning there was no hot water.
3. The propane tank reads 30% & the kitchen stove lights fine.
4. The outside temperature is 17C which is quite normal.
5. The 15cm hot-air flue is at room temperature (it should be hot).
6. The 1cm hot water cupric line was room temperature (should be hot).
7. All the gas lines appear to be intact (no horse urine egg smell).
8. All the water lines appear to be intact (no ponds of water).
9. Black thermostat dial was on the "B" setting (which is normal).
10. Red push-button off/pilot/on dial was in the "on" position.
11. No flame was visible at the sighting glass when we removed the plate.
12. Sparks were visible when pressing the pushbutton green "igniter".

Following directions, we
A. Turned the black temperature dial to pilot
B. Pressed and turned the red off/pilot/on dial to pilot
C. Repeatedly pressed & released the green piezo igniter
D. The pilot candle lit after a few presses of the igniter
E. We held the red pilot dial down for more than 1 minute
F. We stopped pressing the red pilot dial (it popped up)
G. The pilot candle remained steady
H. We slowly turned the black temperature dial to B
I. The flames circled the bottom of the water heater

And that's where we are at this time.
At this junction, nothing seems wrong.
The heater is in its own closet so there are no rain or drafts.

Why would a 50-gallon propane water heater just stop working?

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