More often than not this is due to a pop-up blocker. These tend to block the little window which pops up with the house being built, this window is what loads the Page Builder application so no window = no Page Builder.
Look under your browser toolbar after you click the Page Builder link. Is there a yellow bar with something in it about a pop up being blocked? If so, you should be able to either temporarily allow the pop-up or add the site to your white list.
If not, have you recently downloaded anything like Yahoo! Toolbar (with messenger usually), or have you downloaded a pop-up blocker or changed the settings in one you already had? Almost all pop-up blockers have a white list, a list of sites allowed to launch pop-ups, adding to yours if so.
Even a Firewall Setting can block this window if it has ad blocking capabilities.