。
She was standing in the middle of the room.
Literal
She [topic-は] that room [genitive-の] center [at-に] was-standing.
中央 ('center, middle') is a slightly more formal, written-feeling word than the everyday 真ん中. に marks the location of static existence — the same に that pairs with verbs like ある, いる, and 立つ for 'be located.' 立っていた is the ~ていた form of 立つ ('to stand'); the verb 立つ describes the change from non-standing to standing, and ~ている then describes the resulting state of being upright. Past tense ~ていた places that state in a remembered moment, like a snapshot.