。
She seemed lonely.
Literal
She [topic-は] lonely-looking was.
寂しそうだった is the past tense of 寂しそうだ (seems/looks lonely). The ~そう turns the い-adjective 寂しい into a na-adjective (寂しそうな/寂しそうだ) expressing the observer's impression of her emotional state. The past tense だった places this observation at a specific point in the past.