This has happened to me a few times this week and is probably due to happen any minute now.
The fix is quite simple. Follow the instructions here on the BBC from Virgin Media.
If this doesn't work, you need to reset your IP address. To do this, you need to open up the command prompt by following these instructions: Click start Click run Type in "CMD" and press enter This should open up a command prompt window Now type in "ipconfig /release" and press enter Finally, type in "ipconfig /renew" and press enter Now try to go online to see if the connection is working