Good news it has nothing to do with a shed, bad news it's serious however from a picture and not having a full idea on how the animal it's hard to tell what you are dealing with.
There are two options here, Burn or Scale Rot, scale rot does not present itself overnight and I would think you would have noticed brownish scales starting to appear and would have intervene before it gets to that extreme, so it leaves you with the most likely culprit and that is a burn which means lack of thermostat or thermostat failure.
That's a case for a vet we are not in the mild range here we are in the serious enough that Anti-bitotics will likely be needed along with Silver sulfadiazine.
If dealing with a burn in the mean time and do not have a thermostat, UNPLUG your UTH immidiately (same if faulty) and go to home depot and get a dimmer like this as a temporary solution until you get a T-Stat.
In either case remove the substrate used and keep your animal on clean paper towel and keep it dry.
