。
She was too terrified to speak.
Literal
She [topic-は] too-much frightened mouth [subject-が] could-not-work.
余りにもおびえて口がきけなかった — 余りにも intensifies the te-form おびえて ('being frightened'). 口がきけない is a set expression for being rendered speechless. The te-form shows cause (frightened) → result (speechless).