There are a few solutions worth to try:
- ifdown the network interface and ifup it again
- when perform upgrade of Ubuntu, the system might modified /etc/resolv.conf to include 127.0.0.1 as nameserver. Sometimes this will mess thing up and you can't get online. You can either delete everything inside, or use google DNS: nameserver 220.127.116.11
- if you have bonding, you might want to add dns-nameserver 18.104.22.168 22.214.171.124 under your bond0
- restart your networking: restart networking (this could be different from reboot Ubuntu. Because my connection got fix by restarting the network, while reboot doesn't help at all
If you do not wish to tackle with all this stupid problem, my advice is only upgrade when you need it. As long as your server is running fine and the upgrade is not going to fix any issues, just keep it as it is.