Please Help: Only see front end pages once login…

One of my WPMU site, all a sudden becomes this:

On the front end, a regular visitor, only sees the home page content (hard-coded page, not a post or page from the dashboard); when a regular visitor clicks on any page or post, it jumps to that URL, and shows “No posts found”.

Then I login as admin, and click front end pages or posts, I can see everything. Another thing I noticed is, when sub-site admin login, he can only see his sub site page, not the main site page.

It seems to me, one can only see his own pages or posts that he had created. A regular visitor did not create any page, so he see nothing.

What might have gone wrong? Where do I start to debug?

Thanks a million!