Put it in a bowl and cover with water. Then cover the bowl with cling film and leave it alone for maybe a couple of weeks. By this time it will have begun to dissolve into the water and certainly softened on the outside, using a knife or something similar prise the lump open and allow the water to get everywhere, leave for a few more days. Then put the whole lot into a saucepan and heat it slowly, stirring until there are no more lumps. Don't allow it to boil Leave it to cool in the pan. You may find that it is too wet and therefore a sticky mush, if it's like this warm it again and hold it over the heat for a while again not allowing it to boil. If you have a double boiler that is ideal for re-heating it, but not many people have them so just take care with a saucepan. I'm assuming it is a commercial bought fondant? If so it will nicely go back to what it was. Before it gets too cold tip it out onto some cling and wrap it up, then put it into a plastic bag and seal.
Frisbee