。
She had gone home a long time ago.
Literal
She [topic-は] long ago home [to-に] had-returned.
The combination of ずっと前に ('a long time ago / way back') and ~ていた (here polite ~ていました) gives a past resulting state — at the time being described, she had already gone home and was no longer present. ずっと before 前 stretches the time-distance: 前に ('before') becomes ずっと前に ('way before / a long time ago'). 帰っていた is the resultative use of ~ている in past tense — not 'was returning' but 'had returned and was in the resulting state of being home / gone'.