There is no way to salvage the fudge you made, at least that I know of. There may be many reasons that your fudge turned out grainy. The sugar may be old. If this is so, make sure you have fresh supplies when you make fudge. The fudge may not have reached the correct temperature is another reason. If it doesn't the sugar isn't melted. The fudge may have gotten too hot. If this happens the fudge will turn out grainy too. I always, always use a candy thermometer to make fudge. If I don't it usually doesn't turn out right and I make and have made a lot of fudge. I used to work for a sweet shop and we made fudge all of the time. If the weather is wet it will also sometimes affect the fudge. As you see, there are many factors that can effect it.