Oops, where did my content go?

On one of my WP blogs, to which I haven’t published new stuff in a few years, the content suddenly disappeared.

1. The blog content still is in the database. Thus, from the WP admin section, I can see and edit all posts and pages. I just can’t display them in the blog. So you’d think it’s a theme-related problem, but…
2. Changing themes doesn’t solve the problem, so I guess the cause is something else.
3. When I use a theme I designed for this site (a long time ago), it does display the headlines of the posts, the sidebar and the footer. Everything’s there, except the content (I use the more-tag, so there should be short intros to the individual posts, but there’s just an empty <p></p>.
4. When using K2 theme, the blog’s main page ends with <div id="primarycontent" class="hfeed">, i.e. the sidebar and the footer is lacking in the HTML source. Also, where the content is supposed to be (in the primary DIV), there’s nothing…
5. For individual blog posts, I get the headline, the comments, and the sidebar – basically everything except the text body of the post. This is what the HTML source looks like (using K2 theme) in the section where the Content ought to be:

   <div class="entry-content">
   <p></p>
   </div> <!-- .entry-content -->

6. Perhaps I should add that the blog is in the root folder of my domain (domain.com/), whereas WP is installed in a sub-folder (domain.com/wordpress). But that shouldn’t matter, right?