Well i tried to reply but i must have timed out so I will try again.
So ya i had this problem as well. I have a 2004ZR. It only happend a few times and then i ended up swapping out all the OEM audio for aftermarket stuff. So when it did happen i hit the dash a few times in frustration and yo and behold it was ok again. I was going to take it in under warranty. I paid like 1500 for extra warranty that included the audio but as said above I swapped everything out so…
You could maybe try taking the head unit out without unplugging and give it some jarring to see if its a loose connection inside. The dash comes apart pretty easy once you know where the screws and stuff are. I installed my CF dash and had to take everything out including taking the flip up door completely apart. As Xuper Xero says my350z forum uder the audio and DIY section will have some links and ref to taking the dash out.
Good luck