。
Dawn started breaking.
Literal
Night [subj-が] open-begin-[past-た].
夜が明ける is the idiomatic 'day break, dawn break,' literally 'night opens/clears.' 明ける is a specific verb for night yielding to day (opposed to 暮れる 'dusk fall'). Attached with ~始める ('begin to') for the onset of dawn.