Have you checked your water pressure guage thingy?
My CH did the same thing, and it was a simple case of whilst everything was circulating, the burner stayed on, and it was fine, but once I switched it off there wasn't enough water pressure to fire up the burner, so I no longer had any hot water.
The solution was to turn the little tap thingy until the barometric guage thingy was back in the green zone.
Once done, I again had hot water.
Another option is that, your pump has become skanked up.
Mine cost £120 for a professional prodding of the pump by my friendly Plumber recently (after the first switch off of the CH for the season), and then it was all working again.
Ary.
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***Do not, I repeat Do NOT feed the Trolls! ***
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All Hail the mighty hamsters!
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