Content DISAPPEARS when simply adding normal text for ONE POST ONLY

Maybe the blockquote or it’s containing elements are floated with CSS and by adding one more line, the size of it grows too large to fit in the theme / page layout, and the whole post body gets shifted down or out somewhere where it’s not visible anymore.

You can try the Firebug extension for Firefox, which is an excellent tool for debugging issues like that. But you’ll need to know HTML + CSS to take advantage of it.