Depending on temperatures during the week your peace
lily was forgotten in the car, your plant's roots may not have frozen. Remove the plant from its pot and examine the roots. If they are still firm, white, and healthy looking, the plant has a chance of recovering. Cutt off any roots that are discolored, mush, shrivelled, or carry an odor. Repot into the smallest pot the rootball will fit into using a good sterile soilless mix. If you haven't already, remove any damaged foliage as damaged leaves will not recover. If the roots are all damaged, the plant has no chance of recovery.
Place directly in front of an unobstructed window that receives bright indirect light - a north window is best, although an east window where it gets a few hours of some sun is okay, too. The room should be of average, comfortable temperature. Peace lilies do not like temperatures below 50F.
Water thoroughly, then let the top 1/4-1/2" dry out before watering again. Peace lilies wilt when they are too dry. Unfortunately, wilting causes unnecessary stress on the plant so for an optimal watering routine, the trick is to learn to water just BEFORE the plant wilts.
Finally, patience is crucial. A plant left out in winter temperatures is going to be damaged and stressed. Give it time before you give up on it!
Hope this helps.
Star Dancer