。
She remained silent the entire day.
Literal
She [topic-は] all-day was-silent-remained.
~まま ('remaining in state') attaches to a past-tense verb (黙った, 'fell silent') and freezes that resulting state in place — 黙ったまま = 'as is, having gone silent.' Adding だった ('was') puts the whole frozen-state predicate into the past. The flavor is more static than 黙っていた: ~まま emphasizes that no change happened, that she stayed exactly in the silent state without moving on, while ~ていた describes the ongoing being-silent. The contrast is subtle but real — まま carries a stronger sense of 'unchanged.'