。
She no longer knew what to do next.
Literal
She [topic-は] next [adverbial-に] what [object-を] do [if-good-のか] became-not-understanding.
~なくなる ('come to no longer V, stop V-ing') tracks a change of state from a positive to negative condition — '分かる' (understands) → '分からなくなる' (no longer understands). Useful for describing loss of ability or knowledge. ~たらいいのか is the deliberative ~たらいい ('should / would be good to') wrapped in an embedded question — いい is the more colloquial counterpart of よい (interchangeable). The polite past ~ました places the loss in the past.