Give it a drink, and within hours it's standing tall again, as if nothing happened. That droop is actually useful: it tells you exactly when it's thirsty. Just try not to let it get there often, because repeated wilting stresses the plant and leads to yellow leaves.
Pet note: the peace lily isn't a true lily. True lilies (such as Easter and tiger lilies) and daylilies are extremely dangerous for cats. Peace lilies are still toxic if chewed and can irritate a pet's mouth, so keep them out of reach.
