。
She lay down on the bed.
Literal
She [topic-は] bed [on-に] horizontal became.
横になる literally 'become horizontal' is the standard Japanese idiom for 'lie down.' The に here marks the surface where the lying takes place. The image is geometric — going from vertical to horizontal — which is how Japanese conceptualizes the act, distinct from English 'lie' which keys on stillness rather than orientation.