Unfortunately, the frosting being a fudge means that it will solidify and can't really be brought back once it has begun to firm up. You can try to reheat and melt this over a low heat with a bit of added water, but this will dilute the flavour and may cause sugar crystals to form. It is generally easier to just make more than to try to fix the mix. If you are worried about the frosting firming too quickly as you start assembling, frost a layer to be the top of the cake first and set this aside. You can then know that you have a nice looking layer of smoothly frosted cake to go on top, even if other layers begin to seize up as you are stacking. You can often mould the solidifying fudge frosting to cover layers if necessary, but these don't make for pretty tops to the cake!