。
She became seriously ill.
Literal
She [topic-は] serious illness [to-に] became.
重い病気になった uses ~になる with a noun phrase to express a change of state — she transitioned into the state of having a serious illness. 重い (heavy) modifies 病気 (illness) to mean 'serious' or 'severe' — a common collocation. The plain past なった reports this as a completed event.