Some common web sites don't open at office

[flylite]

Hi,
Does anyone know why some safe common websites don't open on my office PC at work?
When I try to go to these sites, I get that white screen saying "the page cannot be displayed," same as if the ethernet cable were disconnected.
hotmail.com opens, but once I login the next page is the white screen.
gmail.google.com goes directly to the white screen.
They aren't restricted sites because I get a grey screen with a restricted warning for those.
mail.yahoo.com works though

any ideas?
thanks