I don’t know what happened, but Raspbian (the Linux-based OS which runs on the Raspberry PI) somehow lost its network connection. So I’m rebuilding it from scratch.
I’m a Linux system engineer by trade. But even after a bit of investigation and experimentation, I couldn’t get the Pi’s Ethernet interface to work again. The OS saw it and gave it multiple IP addresses for some reason, so I don’t think it was a hardware issue. I tried disabling DHCP and giving it static addresses, and that didn’t solve the issue. Restarting the networking didn’t fix it. Since the installed OS version is over a year old at this point and I want to install some other packages as well, I figured I’d just start from scratch.
As an aside, a story about security. I installed Raspbian on this Pi yesterday, but didn’t change the default password. At the same time, I had forgotten that I forwarded the SSH port (TCP 22) to it some months ago. I woke up this morning and the PI had been hacked and was mining cryptocurrency. It only took a few hours for it to be found, unsecured, on the Internet and compromised. Moral: always change the “pi” default password, and harden the security of your home network and even of these small devices!