When you see this, it means that the NUC was not able to find any local resource from which to boot and is looking for a remote (LAN-based) boot server.
If this is a brand new NUC, you want to insert the media from which you will be installing your O/S and do another CTRL-ALT-DEL to initiate the process.
Hope this helps,
Ok, you need to check the BIOS Configuration and especially the Boot Order. As well, if a device is not showing up that should be there, it may have failed...
Well, if it was booting and working fine one day and not the next, then something has changed. You are sure you didn't change anything in BIOS Setup in between? Possibilities are:
Have you verified that the BIOS is still configured to boot from/through the Optane memory?
Can you boot from a USB flash disk formatted for booting DOS or running/installing Linux or Windows? If you can, then likely something has happened to your boot image (perhaps as a result of the Optane memory.