May be heated frozen or thawed overnight in the refrigerator. For the oven, reheat covered at 375 degrees, in an oven/ roaster preheated for at least 20 minutes, 40 minutes for thawed potatoes (that is a standard size pan, 20 pounds thawed will take double that, approximately), 90 minutes straight from freezer. Stir occasionally as they heat, moving edges to center and bottom to top.
Ready when temp in the middle reaches 165. Turn roaster to 180.
Or heat thawed mashed potatoes in the microwave,covered, stirring every 2 minutes.As the potatoes get hot, they will thicken and return to their original texture.