I had the same problem just recently, I was using a VM with Ubuntu 11.04 as the O.S.
If you go to the network connection manager, you may already have a connection icon at the top.
You should have a display box showing 5 tabs, Wired, Wireless, Mobile Broadband, VPN and DSL
My display showed the only connector as
eth0 and last used as 6 days ago so it was working as yours was.
What I eventually did was simply 'Add' another connection and it defaults to 'Wired connection 1'
simply enter the Device MAC address: (obtained from ifconfig) or, from the
eth0 details already up and make 'Available to all users' and then 'Save'
It should connect automatically, I then deleted the
eth0 that was defaulted in and kept the'Wired connection 1' as the only one in the window.
This worked fine for me on a VM as I said above but as you had yours working once it may be the same and I don't know why it didn't re-connect as the MAC addressing was the same as the newly created one.