Categories Gone — simply stopped showing on my page

I tried adding a page, and did. Then I added the little header to the right of the blog entries, the header that displays the titles of your static pages. When I did that, all the categories disappeared from the page. So I removed the header, but the categories are still gone. Then i disabled two plugins that I had enabled (two that came with the blog, that only required activating, not ones that I uploaded myself). Still no dice. My 130+ categories are not showing. they’re still there, in a sense. You can click on a post and find your way into the category by clicking it on the post, but the list of categories on the right hand side… gone, gone, gone.

Thoughts?