NAD 402 Tuner problem
It's sounds like a chip gave up the ghost. You may want to try
turning in on and off a few times to see if you can get the culprit
to function properly again. Electronics are strange creatures in that you can at times just shake them and they will begin to function properly again. A loose wire here or there, instead of any real problem.
However, if the problem's more serious, given the cost of servicing these products, you may be better off just going to Ebay and buying a used one.
I recently purchased a nice used NAD 402 with original box
for $58. Upon arrival I found that the backlight to the digital display was not functioning, however the display itself was, and the sound of the tuner is amazing. It is nearly the equal of many tuners which I have owned in the past at as much as 50 times the price.
Best of luck to you...