poinsettia dying?

It could possibly be shock. It will have gone from ideal growing conditions, to travelling in wagons in all temperatures, maybe stored in a supermarket warehouse for a couple of days before being bought and put in a house with central heating. If you can keep t in a place where the temperature will be fairly constant, and don't over water it might pick up, but someone else might know better. (I admit I have never had one!)

Mine was exactly the same, it was in a very small pot so I have potted it on and kept it on the kitchen table in the light but no draft and it seems to be responding

They are very susceptible to draughts,that makes them shed leaves & they never return to their former glory, garden centre near here even fasten tissue over the top of the plastic sleeve they are in to protect them on the way home.