Is this a new network or new host? If existing has this happened before?
Open Command.com window and – PING 169.254. . (insert your numbers) do you get Reply?
Also, what version of Windows? Routers? How big is network? Is this just one computer, a few computers or all?
You may try (this might fix the problem):
Check My Network Places – network properties – Local Area Connection Properties – Configure – Advanced – Link Speed and Duplex. If it is not on auto detect change to AUTO DETECT. If it is on auto detect change to 100/Full or 100/half or 10/Full or 10/half depending on speed of your network.
Please answer the questions and try the auto detect feature.