Not displaying posts properly

Has anyone any ideas. I have a site with a custom menu – top level items are all pages (static / dynamic content) except one which is a parent category. Below that one is a sub-category which has several posts linked to it.
If I click on either the top level category or the sub category I see a list of the posts Title and brief extract when I click on the title or the ‘continue reading’ the list of posts disappear and I do not get the full text all I get is a blank page which has just got the text taken from the widgets at the left side bar but displayed on the top left. I have set up a twenty ten child theme – am I supposed to define a template page to be used for displaying the details of the post?