My old NES started having a graphical issue, it seemed like it was drawing sprites but no backgrounds (solid green in SMB as seen in the pic, but different colors with other games).
I was hopeful that this indicated a failure of the VRAM, since (as I understand it) it stores all of the information about how to draw the background tiles (image references, attributes, palettes). I found some replacement chips, removed the VRAM and installed a new one (in a socket, as seen on the bottom of the second pic). Unfortunately nothing changed.
On a whim I replaced all of the electrolytic capacitors, which I'd been planning to do anyway, but this also made no difference.
I saved it as a last resort since they aren't readily available, but I finally replaced the PPU (seen replaced with a socket in the second pic). Success!
Do you know where your DHCP server is? What are you plugging your Debian server's network cable into? I understand that networking apparently worked while you were installing Debian, and it's unclear why that would have changed, but if I were in your position the first things I would suspect would be something with the DHCP server itself (like it stopped running or ran out of addresses), or a lack of connectivity between the Debian server and the DHCP server.
In other words, I'd start troubleshooting with your network, but it's hard to suggest specifics because we don't know anything about your network. One simple thing to try might be to restart your router. For most home networks, that's where the DHCP server will be, and it should start up on boot.