。
She must have arrived late last night.
Literal
She [topic-は] last-night late arrived [should-はずだ].
~はずだ ('should be, ought to be') marks the speaker's confident expectation based on reasoning. Distinct from ~べきだ ('should, ought to' in the moral/recommendation sense): はずだ is about what the speaker concludes is the case; べきだ is about what the speaker thinks ought to be the case. 昨夜 (さくや) is the slightly more elevated reading; ゆうべ is the everyday spoken reading of the same kanji 昨夜.