。
She didn't say anything.
Literal
She [topic-は] anything didn't-say.
なにも + negative verb is the standard 'nothing / not anything' construction — pairing the indefinite なにも ('anything at all') with a negative predicate. Same pattern works for だれも ('no one'), どこも ('nowhere'), いつも with negative ('never'). Note that なにも is itself an emphatic combination: なに ('what') + も ('also/even'), giving 'even anything' which under negation collapses to 'nothing.' The plain past 言わなかった maintains the casual register — a simple, direct observation about her silence.