Hi there
This is a common issue with early NAD players, there are a couple of bulbs soldered into the PCB that light up the display, they tend to go after so many 1000hrs or so. I've a 5325, that I aquired a few weeks back, I've a feeling that 1 bulb has gone in mine, as the display is dim during daylight, but readable in the dark.
Good news it can be fixed, I'm sure any reputable hifi repairer could fix it, I've heard a lot of people opt for an LED replacement bulb, given the life expectancy they carry. Of course the price of repair also comes into it? Does the player mean that much to you that it warrants the price?
Nige