。
She turned down my invitation.
Literal
She [topic-は] my invitation [object-を] declined.
断る is the everyday verb for declining or refusing — softer in feel than 拒絶する (flatly reject) or 拒否する (refuse outright), but firmer than お断りする (the keigo equivalent). In Japanese, declining an invitation is often phrased indirectly with hedges like ちょっと... or 都合が悪くて... — the bare 断った here makes it clear the refusal was straightforward, perhaps even abrupt. Note the verb pair 招待する (invite) and 招待 used as a noun without a する suffix, with を marking it as a direct object of 断る.