Fudge already set, can you soften and re-set it?
My understanding of the fudge process is that you are aiming to make a supersaturated solution of sugars at heat, which, when cooled, will cause the sugars to crash out of solution all at onece into the fine crystals characteristic of fudge. This is the reason you need to remove sugar crystals from the sides of the pan - so that they don't act as nucleation points, allowing bigger crystals to form.
There are a few different sorts of fudge, some of which will be easier to fix than others. Traditional fudge is the fussiest to get right, but also the easiest to fix if necessary. So called &quot;quick&quot; fudge made with condensed milk or marshmallow fluff is easier to get right, but not as easy to fix, and does not rely on so heavily on sugar crystallization, but rather on protein precipitation acting as .
In principle for traditional fudge you could re-heat it by adding more liquid so that the sugars dissolve. You would then need to evaporate the excess liquid (but don't exceed the soft-ball stage at 237 F/114 C) to recreate your supersaturated solution. Whether this will work or not will likely depend on your skill at maintaining the temperature below the point above, so as to neither burn the fudge, nor turn it into toffee.
For quick fudges, there is some protein content in there from the milk or the gelatin in the fluff, which affect these. You can't really re-dissolve proteins that have precipitated and aggregated, so you will not be able to melt these ones effectively.