VirtualBox guest has no network

It appears that your guest only has a loopback interface for whatever reason.

Try entering the virtual machine settings and see if the VM has a network card installed.

If that is not the case. And you have recently changed the VM's mac address then I would look at this thread as the user seems to have the exact same problem as you.

https://forums.virtualbox.org/viewtopic.php?f=7&t=43090

specifically this "each time you give a new MAC address (in VBox guest settings) Debian and Ubuntu guests udev assigns a new eth number edit /etc/udev/rules.d/70-persistent-net.rules or delete the file: it will be recreated @ next boot"