Webpage Displays Totally Blank

I have a web page that I have been using for a long time and looks fine on the designer page display, but displayed as all blank when using the "preview current page" mode or when viewed in a browser.

The issue was probably due to something I did when changing things in the Page & Layer Gallery but I didn't see anything unusual there. I fixed it by loading a recent backup, so the problem is not urgent. However, I would really like to know what I could have done to cause that problem. Any ideas?

Usually happens if everything is on the MouseOver layer.
Occasionally when a Placeholder has code that is not correctly terminated.

Acorn