I am assuming by \"Peace Lily\" you mean the houseplant with the broad green leaves and white spathe-like flowers. This plant thrives in low light situations so too much sun can cause it to turn yellow. It also needs good drainage so too much water or sitting in a dish of drainage water can also cause the leaves to turn yellow. You may want to repot it into a container with large drainage holes if the plant has grown too large for it's pot or if the drainage holes are blocked. This is a tough plant so I am optimistic that you can correct the problem and it will recover. Keep growing, Marianne Binetti