。
She lay face-down on the bed.
Literal
She [topic-は] bed's top [on-に] face-down was-collapsed.
うつ伏せ ('face-down, prone') is a noun describing body posture — its opposite is 仰向け ('face-up, supine'). Used adverbially here with に ('in a face-down manner'). 倒れる ('to fall, collapse') with the resultative ~ていた marks the ongoing state of having fallen — she was lying there in that posture, having collapsed into it. The full image is heavier than 'lying' or 'reclining' — 倒れる suggests collapse, exhaustion, or grief.