。
Where on earth has Yoko gone?
Literal
Yoko [topic-は] on-earth where [to-へ] went-[regret-てしまった] [nominalizer-の-[probably-だろう]].
一体 ('on earth / the hell') is a strong intensifier of questions, indicating bewilderment or exasperation. のだろう self-questioning 'I wonder.' 行ってしまった ('went [regrettably]') suggests she's gone somewhere and won't come back — the ~てしまった softening the concern into frustration.