Web addresses were inveted for convenience. "Real" web addresses are actually a series of numbers - not easy to remember so someone came up with the idea of using more memorable text based names. The thing is something has to translate these text names into the numbers that the internet understands. This is done by things called DNS servers. These are special servers each ISP has wich carry the translation tables. Now I'm sure you can imagine the complexity of propogating all this information to every ISP in the world. In general it works well but sometimes fails. It could be that your ISP has a (temporary) problem with its DNS. This would result in a lookup failure. To test this do a domain name lookup and make a note of the "real" address. This will be in the form xxx.xxx.xxx.xxx in numbers. Enter this in your browser's address bar. If the site pops up then you've proved a DNS error. If it doesn't then there are a number of possible reasons - if it is a new site then propogation around the world can take a few days - if not new then most likely the host server is down. The actual error message would help you here.