My nephew built a new system for my niece using a Z690 Classified.  It has a very strange problem.  If you attempt to restart, the screen goes black and doesn't come back up.  The power button has to be held to do a hard reboot.  My nephew said he searched everywhere for an answer and couldn't find one.  Not sure just how "searched everywhere" he actually did it, but I thought I'd bring it up to those on here.'
 
I wasn't there when he put the system together, so have no idea how well he did it, but he previously built his own system, and it works as it should.

I'll let him know.  Thanks for the suggestion.

👍

So, found out from my nephew that the "set BIOS back to default" didn't work.  

Okay, I'll let him know that he should try the newest BIOS version.
 
My BIOS is fairly "old", but my machine seems to fun fine, so would you say it's best to "leave well enough alone", when it comes to mine?

Interesting, I have the exact same issue with this board and have had it since I built my system. I looked everywhere online; this is the very first time I've found someone with the same issue. 
I don't really have a solution but here are my notes: 
I have a 13700kf and a 7900xt with 2 sticks of 6400mhz GSkill RAM. When I initially built this system I mounted my GPU vertically and that absolutely refused to boot at all. Mounting my GPU horizontally allows the system to boot, but like you, if I restart I get a black screen and no display. Through some experimentation I found that it's possible to update Windows if I just let the black screen sit for a while. Prior to that I thought I had no way of updating Windows. Also, on restart the built-in speaker beeps five times, which indicates a video error. My system is also unable to return from sleep properly. I get a black screen then too. 
I've had no real problems other than a couple of errors on booting that seemed to be random (a CMOS checksum error, and "Memory has changed!" even though it didn't).
I tried a whole bunch of BIOS settings and they didn't help.
Thanks to you I at least know that the issue is with the board and not my GPU.

SAD_GOD_GREEBA, I hope you find the solution to your issue.  Also, when I first built my system, I also was running a vertical GPU mount.  I was getting so many errors and many on here helped out trying to figure it out, only to FIND out that it was a problem with the cable I was using.  That's why, when I put my GPU straight into the board, it worked.  A few months ago, I did end up getting the right cable and now I'm running a vertical GPU again, with no issues.
 
For my nieces system, my nephew believes she has the latest BIOS.  But, B0baganoosh, you mentioned updated nvidia firmware.  I can tell him to try that.  It's very weird.