Recent convert to Ubuntu after teaching Comp Sci using windows. Two machines connecting through ADSL modem. Just upgraded the Toshiba A100 laptop to 12.04 and now it says network not connected. My guess would be laptop drivers BUT I don't know how to diagnose or find/install such drivers.
Are you able to use the network in some other context? Booting Windows or another version of Ubuntu or Linux, perhaps?
What version of Ubuntu did you upgrade from? Was the network working before you upgraded?
Is this a wired ethernet or wireless connection? (I'm guessing wired, but safest to ask.)
What have you already done to determine the network is not working? Have you tried to start the network by clicking on the