Unhealthy Peace Lily
It sounds as though this peace lily has been under stress for a long time. Even if given proper care it could take a long time for it to recover. Although peace lily tolerates low light conditions fairly well, it will grow better in bright indoor light. Keep it warm (at room temperature or slightly warmer) and moderately moist. Allow the surface of the soil to dry slightly between waterings. You may want to check the root system of this plant to see if it has been damaged in some way. Pull it out of its pot to see if the roots are firm and healthy white or light tan. If they are soft, mushy, or darkened, it's an indication that they have rotted, either from overwatering or disease. Even if the roots are healthy, this would be a good opportunity to repot the plant in fresh potting soil if it hasn't been repotted recently.