Log into the administrative interface for your router and see if there is anything wrong with the configuration there. Make sure it's actually detecting your desktop that can't seem to get connected.
Also, check the back of the desktop and make the link lights are flashing on the network adapter, indicating that a connection is present and active. If there are no link lights, unplug the network cable and plug it back in.
Thanks followed instructions but still could not get on the net. I was told of a local lad who could fix any computer so I contacted him. He came to my house took my tower away and bought it back the next day fixed. The problem was the net card and he charged me £10 for it. The other bloke I got to look at it who said he could'nt find anything wrong owns a computer repair shop. I know who my repair man will be now if I have anymore problems.