After I upgraded from Ubuntu 11.10 to 12.04 I could not access the internet through my router wired or wireless. Here is the symptoms and how I solved them:

After the upgrade I noticed that XBMC was missing, while this may be unrelated I wanted to reinstall it using apt-get install xbmc xbmc-standalone in the terminal. This echoed the following message

Something wicked happened resolving ‘dk.archive.ubuntu.com:http’ (-5 – No address associated with hostname)

So I googled around and found that the problem lies with the wvdial package, which is uninstalled during the upgrade. So to solve the problem I simply had to reinstall that package. As I could not access the internet wired to a router or connected wireless – I had to use my 3g dongle. This turned out not to be a problem.

In the terminal write

sudo apt-get install wvdial

After the installation I tried to connect to the internet through a wired and a wireless connection, and had no problem.

If you are not as lucky as me to have a 3g dongle, simply download the package to a USB from here http://www.ubuntuupdates.org/package/core/precise/main/base/wvdial, and installed it using this command

dpkg -i wvdial_1.61-4build1_i386.deb 

Update: If this is not working, you might want to try to add a DNS server. Thanks to alan for point this out.