If you've recently migrated a Ubuntu virtual machine file from one computer to another, or from one virtualization product to another, you might have discovered that networking inside the VM was no longer working.
As described in this Parallels KB article, what's probably happened is that the random MAC address generated by the virtualization host has changed. The file:
/etc/udev/rules.d/70-persistent-net.rules contains a record of the matching MAC address from the previous host. When Ubuntu networking starts, a new interface, perhaps
eth1 gets created, but is not listed to start automatically.
The fix is to edit
/etc/udev/rules.d/70-persistent-net.rules to contain the MAC address from the new virtualization host's virtual network hardware.