Do you have a picture?  Which cable was melted power to riser a lot of times has adapters, wondering if adapter or truly PSU cable melted.

Either way I would touch other cables like the one that melted and if you feel heat git rid of cable.  I have only melted 1 cable and it was due to "bargin" psu adapter it used.  Ever since I stay away from low quality PSU and adapters.