is that you're being prompted for a logon before you can access the site page that you want. This is a normal procedure for sites which operate a registration system.
If the login prompt has only recently started to appear it could be that the site operators have instituted a registration system. It would be a little odd to ban people in this way, but it may be the case. In any event, you can't gain access, so it might be an idea to forget about it and move on.