。
She kept silent all day long.
Literal
She [topic-は] all-day was-silent.
黙る ('to fall silent / shut up') is a change-of-state verb — the moment of going quiet — and 黙っている is its resulting-state form, 'is in the state of having gone silent' = 'is silent.' 一日中 stacks the same ~中 frame seen in 一晩中, marking continuous duration through a span. The combination 一日中黙っていた pictures her settled in silence for the entire day — not just briefly, but as a sustained state.