Try opening internet explorer and going to www.google.com
If this also produces an error, go to START -> Type "CMD" in the search box and hit ENTER
A black box will apear.
Type "ping www.google.com"
If it comes back with errors, it is your internet connection. You can be connected to your home network whilst your internet is not working.
Are you on windows 7?
If so try this:
There should be a network icon at the bottom right of your screen, a computer icon. Right click it, select "open network and sharing center"
You should have a network that says LOCAL AREA CONNECTION. Select this.
Click on DETAILS...
You should have an IPv4 Address that looks similar to "192.168.1.3"
Write down this address.
Close the current screen. You should be on the LOCAL AREA CONNECTION STATUS screen again.
Double click the INTERNET PROCOTOL VERSION 4
Select USE THE FOLLOWING IP ADDRESS:
Now remember that IPv4 address i told you to remember? Well put that down in the IPADDRESS field, except add +15 to the number.
Eg. original: 192.168.1.3
Make it: 192.168.1.45
Subnet should be: 255.255.255.0
Default gateway should be the same as your IP but with a 1 at the end.
As for the DNS - Log in to your router/modem and in the summary or overview section it should list the primary and secondary DNS ip addresses, find them and fill in the remaining 2 boxes.
That's it, you're done. Enjoy your internet.