If you entered it manually, you might have entered a wrong number? I had something like that happen to me and when I checked my settings, I entered the wrong number in one of the fields. Make sure all of your info (Gateway, DNS, IP addresses) are correct and you should be fine.
If it's set to automatic, then it might be your connection. Contact your ISP. I'm no network specialist but I'm sure someone with more knowledge on it can help ya.