This has been a recent case with Naso tangs mainly from Sri Lanka in which they are healthy then deteriorate in this fashion. Both you and LFS may not know origin as wholesalers may or may not even know. This fish does not look like a new acquisition and often as with mine a while back, flukes may trigger this without warning signs as they have a velvety texture to them already.
Looking at the retina damage/dots is a first sign of fish becoming moribund why I requested pics. Im sorry firstly to see fish in this condition and to reiterate what Jay stated- Fish appears Moribund which is a condition similar to comatose state and fish may or may not make it. Adding aeration with air stone may help its breathing However if fish is inactive and not eating as its doing, " hope for the best but expect the worse outcome".
If you have a quarantine tank, you can set up fresh seawater, airstone and apply Prazi as I had to do with mine and hope for a miracle turn-around