Complex sentence: Apparently, upon retiring for the evening, the Snyders had lit the burners on the range oven to heat the rooms; but during the night the flame had been extinguished, possibly by a draft.
Split into simpler sentences: Apparently, upon retiring for the evening, the Snyders had lit the burners on the range oven to heat the rooms. But, during the night, the flame had been extinguished, perhaps by a draft.