I recently moved my Pi-Hole virtual machine to a different back-end, with the result that DNS resolutions started failing across the board. Upon closer inspection, I noticed that Pi-Hole had not realized that the network interface had changed, from enp1s17 to enp1s0, which you can see in the web management page tab: settings / system. The result was that it could not communicate to the outside world.
OK, found the problem, now how to fix? The web management page does not have anywhere that you can click to update this information. An online search said to run the command pihole -r, which, for me, did absolutely nothing.
To get it working, I edited two files:
Let me add, all this is provided as a means for me to remember and document changes to my systems. Follow at your own risk.