。
She didn't pay me back the money.
Literal
She [topic-は] me [to-に] that money [object-を] paid-not [for me-てくれなかった].
The negative auxiliary ~てくれなかった ('didn't do for me') sharpens the sense of grievance — the speaker isn't just stating that the money wasn't paid, but registering it as a withheld favor. The polite-but-formal お before お金 raises the noun's register slightly (compare 金, the more bare term) — Japanese often softens references to money, an awkward topic, with the polite お prefix.