。
She was overflowing with joy.
Literal
She [topic-は] joy [with-に] was-filled-and-overflowing.
満ちあふれる is a compound verb pairing 満ちる ('to be full') and あふれる ('to overflow') — two near-synonyms stacked for emphasis, giving 'full to the point of brimming over.' The に on 喜び marks what fills her — the grammatical subject of being-filled, with the topic 彼女 as the container. The pattern Xに満ちる ('be filled with X') is productive: 自信に満ちる ('full of confidence'), 期待に満ちる ('full of anticipation'). The ~ていた form gives the sustained-state past.