You typed the address correctly but your connection suddenly dropped or the server hosting the website is temporarily down.
If you're a Mozilla Firefox user, TryAgain add-ons is gonna keep trying to load a webpage when the server is not found. Currently, TryAgain works with Firefox version 3.0 - 3.5.*
This is very convenient so that you don't have to manually click refresh or click the try again button. You can set the amount of timeout and maximum tries. I personally like this setting :
Click here to get TryAgain for Firefox.