。
Her personality has completely changed.
Literal
She [topic-は] completely personality [subject-が] changed-ended-up.
A canonical 'something happened to her' construction. The double-particle pattern 彼女は性格が変わる puts the experiencer (her) as the topic and the changed quality (personality) as the grammatical subject of the change-of-state verb 変わる. すっかり ('completely') intensifies the totality of the change. ~てしまう adds finality and a slight tinge of regret or disapproval — 'unfortunately, irreversibly, she's now a different person'. The whole sentence reads as a quietly dismayed observation rather than a neutral statement.