Please provide more details about the NIC you are using. There are several common reasons why a NIC cannot connect, and these include the correct kernel module not being installed, an incorrect configuration of the card, or simply that you need to enable eth0. If you can provide more details about your hardware you likely will get better responses.
In the meantime, you might try this: As root, run these commands and post the results
ifconfig eth0 down
ifconfig eth0 up
The first command lists your PCI devices, the second one lists the kernel modules you've got loaded, the third one shows your current NIC settings, the 4th and 5th ones bounce eth0, and if that works, the final one will show that you've got a working IP address associated with the NIC. If so, you should be able to connect. Note that this is just a generic solution, and assumes that you just need to bring eth0 down and back up again. Without more details about your particular setup, it's difficult to offer any more specific suggestions. -- J.W.