When the cat died, Diana became completely hysterical.
Literal
Cat [subject-が] died time, Diana [topic-は] deranged-state [to-に] became.
錯乱状態 (さくらんじょうたい, 'state of derangement/hysteria') is a clinical-sounding compound: 錯乱 ('confusion, derangement') + 状態 ('state'). になった ('became') marks the transition. The sentence is straightforward emotional narration — the loss of a pet triggering an extreme reaction.