。
She's been busy ever since yesterday.
Literal
She [topic-は] yesterday [from-から] continuously busy.
ずっと ('continuously, all along') paired with から ('from, since') traces a state running uninterrupted from a past starting point right up to now — the Japanese equivalent of English 'ever since.' The bare i-adjective 忙しい sits as the predicate without だ or です, marking this as casual register. Because Japanese present-tense adjectives can refer to states extending into the present without dedicated perfect-tense morphology, 忙しい alone covers what English needs 'has been busy' to express.