BTW you don't need to reinstall Ubuntu. The issue is to do with the persistence model implemented by the udev discover process because your MAC address has changed. The easiest workaround is to edit the file /etc/udev/rules.d/70-persistent-net.rules and delete the lines including "'# PCI Device ..." to end. The net generator will recreate the correct entries on the next reboot.

If the networking on your virtualbox/ubuntu instance suddenly stops working when you accidentally change the mac address, this link is your salvation.

virtualbox.org :: View topic - No network interface in Guest Ubuntu