Have you checked: Fuses, breakers, did someone unplug it?,and or are all the interconnects still properly plugged in. As a former electronic technician these simple things fix things about 10-15% of the time.
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If it powers up but no sound and all settings are correct most likely internal fuses have blown.
Also try unplugging it for 30 seconds and then plugging back in and trying again. Sometimes computer controls in modern gadgets get into an abnormal state and require a reset. Removing power at the source is often the cure.
In addition to the suggestions above, test a source which you know has material for all the channels, i.e. test tones.
The response of the authorized repair facility is puzzling to me. The 7X5 series of NAD receivers has been out for what, maybe six months now? It sounds to me like this repair facility really isnt interested in working on your receiver could be for a number of reasons and none of which may have to do with you or your receiver. Also, I imagine NAD would be interested in hearing about your experience with this particular authorized repair facility.
I have a 751 which suddenly stopped working correctly about two years ago. After running a series of trouble shooting tests, I eliminated all the other possibilities and sent the receiver off to The Service Bench in Norwood, Mass. The problem was simple, the AC3 processing chip failed. From what I understand, it is not uncommon for these processing chips to fail after about six years, regardless of the brand name on the receiver. I had owned my receiver seven years at the time mine failed.
Get the unit back to the US distributor and let them get it fixed. You will pay the shipping cost to get it there, but it still is the way to go. These authorized' factory' service places are a rip off in that they get paid very little for warranty service so they make it up on the out of warranty stuff. Often the distributor/manufactuer are very slow to pay, leaving little if any profit in being an 'authorized' place. I understand that the business practice of many authorized factory service places is to demand payment up front from the distributor/manufactuer before they will fix it. That assures a no hassel profit.