。
She seemed lost in thought.
Literal
[topic-は] reverie [in-に] is-absorbed [seemed-ようだった].
もの思い (lit. 'thing-thought') is a poetic noun for sentimental, brooding, or wistful contemplation — distinct from 考え事 ('thinking about something') which is more practical. ふける ('to be absorbed in, indulge in') typically pairs with abstract states: ふけっている marks the ongoing absorption. The past form ようだった ('seemed') projects the speaker's inference back in time — observed her looking lost in thought, made the inference at that moment.