According to About.com, dry fudge is often caused by improper beating or cooking. The hard, crumbly result can be salvaged by melting it down, recooking and rebeating.
About.com recommends adding the fudge and 1 1/2 cups of water to a saucepan on low heat. The fudge should be stirred until it melts and combines with the water. Scraping the sides of the pot during this process prevents sugar crystallization, About.com notes. Once melted, the mixture is heated on medium until it reaches the temperature called for by the recipe.
The cooked fudge is beaten only until it thickens and begins to lose its gloss. Overbeating hardens the fudge.