dmesg | grep -i eth
If you see kernel messages mentioning network hardware, then the interface is being detected by the kernel. It may just be that you've messed up your user-land configuration.
Is your guest OS Ubuntu Desktop? If so, check your network settings in the Network Manager applet. Create a new entry there for your interface, if one doesn't already exist.
If you're not running Network Manager (or some other graphical network config applet like
wicd), then your network probably needs to be configured in the
/etc/network/interfaces file. Presuming your network interface is named
eth0, and presuming your LAN has a functioning DHCP server, the following entry in your guest's
interfaces file should do the trick:
iface eth0 inet dhcp