Here’s what Maida has to say about rescuing fudge batches:
 If you're fudge turned to sugar, or if it is stiffened in the saucepan,
 or if it crumbled when you cut portions (because you beat it too long),
 add 2 tablespoons of cream and stir over very low heat (cutting up the
 fudge with a wooden spatula while you stir). Cook and stir only until
 warm and slightly softened but not until hot or thin. Remove from heat,
 beat again until smooth and thick, and them turn it out of the saucepan
 again. (Some cooks think that this twice cooked fudge is creamier and
 more smooth than otherwise.)
 If you have the opposite problem of fudge that did not set, you didn't
 cook it long enough or you poured it too soon. If you think that you did
 not cook it long enough, add 1/4 cup milk or cream, then cook and stir
 constantly until it reaches 236 degrees again. Cool as above, and beat
 again. But if you think that you cooked it enough and simply poured it
 too soon, transfer the mixture to a marble, tile, or formica countertop.
 Then squeeze it between your hands and knead it as though it were bread
 dough until it is firm enough to hold a shape. Then roll it into a
 sausage shape, or form it into a square about 1 inch thick. Let stand for
 just a few moments and them slice the sausage shape or cut the square
 into portions.
 Unless someone wants to lick the pan, the fudge that remains can be
 scraped out, kneaded with your hands until it’s smooth and creamy, and
 rolled into marble size balls