I experienced some issues with my NIC after upgrading to Windows 8. The device was enabled in device manager without any problems and the drivers were up to date.
When I entered the network connection properties, I saw that IPv4, IPv6 features etc. were disabled and could not be activated.
When I disabled the virtual switches in local Hyper-V Manager, it suddenly worked.
I had simply looked at the „wrong“ network connection, because the names of the connections can be quite confusing.
If you do not need any Hyper-V NIC devices, just delete them, and you can work with your default LAN connection, in most cases called „Ethernet“.
If you need virtual switches in Hyper-V, enable them and then have a look at your available network connections again.
What had happened in my case:
Some development IDE like Visual Studio or Eclipse had installed a VM in Hyper-V which I had not recognized before (some sort of device emulator).
The Hyper-V Wizard had moved my static IP configuration from „Ethernet“ to „MyNewHyperVSwitch“, so this means the whole network configuration (for the physical PC) has to be done in this virtual adapter. The wizard had not moved my Gateway IP setting, it was just empty.
After correcting the Gateway value in the new virtual LAN adapter, everything worked again.