Peace lily can tolerate a lack of light so it generally does well indoors. I think from the symptoms you described the plant may be suffering from wet soil conditions or root rots. Indoor plants need much less water than the same plant kept outdoors. When the soil is kept too wet roots can't get oxygen and die. This leads to rots and usually to the loss of the plant.
To try and save it, turn the container on its side and slide the root ball and soil out. Examine the soil to see if it is soggy wet or not. It may be that the soil is now drier. Examine roots. They should be creamy white colored, not black, grey or brown. If the roots are rotted stand up the root ball and plant and cut away the outer parts (outer inch or so) of the root ball with a knife. Then repot the plant with fresh garden soil.
Keep it moist but not at all soggy for several weeks to give it time to recover. Trim away dead leaves, and keep the plant in a bright area but out of direct sun. Time will tell if it can recover.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips