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When the breaker flipped and everything went pitch-black, the baby was scared and started crying.
Literal
Breaker [subject-が] fall-and pitch-darkness became time, baby [subject-が] scared-[and-くて] started-crying.
A compact narrative with a stacked ~て chain. ブレーカーが落ちる ('the breaker falls') is the standard collocation for 'the circuit breaker trips' — Japanese uses 落ちる here in the same 'drop out, fail' sense as English 'go down.' 真っ暗闇 ('pitch darkness') is an intensified form: the prefix 真っ ('pure, utter') + 暗闇 ('darkness'). 怖くて is the ~くて form of い-adjective 怖い, chaining it causally to the next verb — 'being scared, [the baby] started crying.' 泣きだす is a compound verb 泣く ('cry') + 出す ('begin, start') — 'to suddenly begin crying.' The ~出す suffix productively means 'break into X-ing.'