First thing is first, if you can go online using that computer using another interface like wireless one, start up synaptic package manager and then search for kernel and then install the latest version(version 220.127.116.11 last time i checked).
Restart your computer and the new one will be on the boot up list, select it and boot up using it and see if the problem is still there.
If it is, then we need to know a thing or two about your hardware, you can not give enough info here, the more you give the better.
Can you open the terminal, type "su" then press "enter" then enter root's password followed by pressing enter again to gain access to root's account and the type these commands one at a time and paste their output here if you can.
cat /var/log/syslog | grep ifplugd