A number of solutions I can think of off the top of my head:
Sometimes DNS changes take a long time to propagate. Try again in a day or two.
Clear your DNS cache if you're in windows (open a cmd prompt, and type "ipconfig /flushdns"). If in doubt, reboot.
Clear your local DNS server cache, if you're using one (such as a Microsoft DNS Server at work).
If you're using firefox, this still won't help. You need to close FF and open it again after a DNS change, since it caches DNS responses.
Make sure you're getting the response you want with an "nslookup" command. Open a cmd prompt, and type "nslookup". At the new prompt, type www.yourdomain.com. If it's wrong, try yourdomain.com (without the www). See what response you get.
Check with your registrar (godaddy, whatever), that you're using the right DNS servers, and not something like "PARK12.GODADDY.COM" or something.