Why does my Moto G5 screen show a yellow triangle within a black circle with a black exclamation mar

Mobile been working very well since I purchased it in June but, last evening it shut off and showed a yellow triangle within a black circle, with a black exclamation mark in it. A tiny red line along the top of what seems like a portion of a the outer edge of the circle, some .5% of the circumference, or outer wheel.

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