Site Builder checks widget content for errors when you save it, but code in an HTML Editor or Template Code widget can still break a page. Then the page may not show, or the Site Builder buttons may not work.
Add /repair=true to the end of the page's address. For a page at yoursite.com/music, go to:
yoursite.com/music/repair=trueThe page shows each widget's title without its content, marked REPAIR MODE. Start Site Builder, and change or delete the widget that causes the problem.