Check to make sure the address you are trying to acces doesn't have a slash at the end (which would mean that you are asking for a directory listing instead of trying to read a page).
Make sure that you are accessing the front page or index of the website, not one of the deeper pages -- their address may have changed since you last used it. Don't use a bookmark to access, use a typed in address or BigBadBugga's link.
Clear your cookies for the site.
Make sure cookies are not disabled for that website.
Do a reload to clear your cache.