“What could be wrong with my Peace Lily? I’ve had it for multiple years and over the past month it has started drooping.”
Is the Peace Lily receiving the right amount of water? Be sure you only water when the soil starts to feel dry. Does the container have drainage holes? The plant should not be in standing water. If the soil dried out previously (you would see it shrinking away from the container) you will need to soak it in water for a few minutes so it can absorb water again.
Is the window drafty? Is there a heat source nearby? Sharp temperature fluctuations in either direction could cause stress to the plant.
How much light is the plant receiving? Too much light can cause these symptoms in Peace Lilies.
Have you checked for pests? Look closely on the undersides of the leaves and in the soil for tiny pests. Also notice if there is any sticky sap.
How humid is this site? Humidity levels are much lower this time of year and that can take a toll on tropical houseplants. If nothing above checks out, consider moving the plant to a more humid location such as a bathroom and see if the situation improves at all.