Menu Items Return a 404 Page not Found

Hi,
I am using WP V3.4.1 with Standard Theme 3.0 on my blog at https://www.leadershipconnexion.com.

I recently found that my categories return a 404 page when I click through on them, although all my posts are linked to a category. One catagory though – Social Leadership – is linked to a spesific post and does not show the other posts linked to the category.

I do not know what could have gone wrong since it worked fine and I did not make significant changes to my blog.

Please help!