Naso's tend to get sick and peter out and die.
Just to be safe make sure you feed it brown algae which is her natural food.
I would recommend that you soak it in selcon and garlic extract which may help.
They also like highly oxygenated water and there may have been an 'event' that reduced the oxygen in the tank for a period of time. Try placing a powerhead just below the water line to add oxygen. If you see her swimming in the current that may have been the cause of her 'illness' Happy New Year