Ethernet stopped working on my PC, and a USB adapter doesn’t help

My wired internet worked normally until about a week ago, then suddenly stopped. The Ethernet port on the PC no longer shows any link light. I confirmed that the cables and router ports work by testing them with another computer and an Xbox. I also bought a UGREEN USB Ethernet adapter, but that does not work on this PC either, even though the adapter works with the same cable on a laptop and Xbox.

I've already tried flushing DNS, releasing and renewing the IP address, rebooting the PC and modem, switching cables and router ports, resetting the network adapters, disabling the proxy, reinstalling both the motherboard LAN and UGREEN adapter drivers, and trying different USB ports. Releasing and renewing the IP briefly restored the connection for about 20 seconds once, but it stopped again. What else could be preventing both the built-in Ethernet and USB adapter from working?