If it is not listed under adapters or unknown, then it is not on ... or not functioning.  Check BIOS Setup (F2), Integrated Devices, and make sure that it is enabled.  If so, then the NIC has probably failed.  You might try clearing the NVRAM using the jumpers on the motherboard.  You might also try booting to a "live" CD to see if it works from there before giving up on it.  If that doesn't work, then just get yourself a network card.
Hello,  As the title states, I'm having a problem when trying to install Windows 7. Usually, it all works when I install Windows 7 on this Poweredge T110 server. But, for today, it doesn't seem to work anymore! Help guys..
Windows 7 missing network adapter drivers (One Broadcom NetXtreme Gigabit)