We've got an aging Dell T300 server used as a utility box. It's been dependable despite being neglected--it was recently found to still be running Ubuntu Server 13.10, with an uptime approaching 1 year. So do-release-upgrade was ran on it and was mostly successful.
The box had it's eth0 and eth1 onboard NICs bonded into an LACP interface. After the do-release-upgrade it appears that eth1 has dissappeared. The device no longer registers during boot, and does not appear during in lshw, lspci, or dmesg--but it's twin eth0 is still present.
I've jumpered the NV_RAM CLR jumper and have checked that both NICs are enabled in BIOS, but otherwise can find no sign of the 2nd NIC registering.
If you see the NIC in the BIOS, your issue is possibly with the OS. In addition, the timing following an OS upgrade is suspect.