Today 1 of 2 identical XCP-NG 8.2.1 servers … after restart … had no network interfaces listed and no interfaces are present. I’ve tried "Emergency Network Reset’ and no joy.
There are VM on the local storage and I’d prefer not to reset the local storage. What options do I have? I’m OK to do a reinstall … or upgrade if that would work … I just don’t want to reset the local storage.
Any guidance would be greatly appreciated! 
SSH into the host and see if the network cards still exist and if they can be set up again:
You could also boot to a linux rescue disk and see if the interfaces are working. Also you can get a terminal from a keyboard and monitor attached to the host, assuming that the OS comes up far enough to get there (assume it does since you did a network reset).
Are the cables good? Are the ports on the switch good? SFP modules good?
Can you assign a different interface to an address? (assuming SFP+ for main connection and 1gb onboard available).
Does the server have IPMI or some other management interface, and does this share a connection with one of the main interfaces? If yes to both can you see the management functions? If management is on a separate interface, can you log in and look to see if it reports anything useful?
Thanks everyone for all your help. All your recommendations have been very helpful!