There's problem on the pcDuino v3B that occurs if you update or upgrade the Ubuntu operating system. WiFi networking still works, but the wired ethernet (eth0) is disabled.. It seems to be a Ubuntu problem, not a pcDuino problem as it has only been happening to people since recent Ubuntu updates and it seems to have happened to various distros.
I believe there is a way to correct it by editing a system file, but I'm not sure I have the patience at the moment to go looking for it. So here's a quick fix that worked for me.
Create a file in your Desktop folder and call it something like Start-Ethernet and copy the following 2 lines into it.
sudo killall -9 NetworkManager
Then make the file executable (you can do this by right clicking the file in File Manager then choosing Properties > Permissions and click in the check box for Make Executable)
Then when you click the file on the desktop, it will stop your network, then restart it. Instead of teh WiFi symbol on the right end of the panel, you will see two arrows, one up - one down. This is the icon to show Wired Ethernet is connected.
It is probably not the correct way to fix the problem, but it is simple, and it works.
When you reboot the pcDuino though, you will be back to Wireless Networking. Simply click the file again and wait until it swaps over to Wired Ethernet.
Obviously, if you don;t have a network cable plugged in you can ignore this. But if you don;t have a WiFi router and have to rely on cables, this might just help you.