I installed ubuntu 12.04, dualbooted with windows 8. I access Internet via proxy server using ethernet hub. It works well with Windows 8 but in Ubuntu, is says wire disconnected. How?
Wire disconnected sounds like the driver for your network card has loaded but the firmware is missing.
Many cards rely on extra software to be loaded to actually "do the work" for them, this makes producing the cards easier and cheaper and also makes upgrades easier as it's just a software upgrade rather than flashing your cards bios. This type of card requires that you obtain the additional software (known as "firmware") and putting it in the correct place (on Linux this is normally /lib/firmware).
Look in /var/log/dmesg to see if there are any messages relating to your network card as it may tell you what file is missing.