I just upgraded the blog software to Wordpress 1.5.3 due to a security fix. But then when I clicked on the comment boxes or on a post title, I got an error saying 404 page not found. The whole upgrading process is very opaque to me, even as a computer programmer, so I had no clue what was going wrong. Fortunately, I discovered this post on the Wordpress support forum indicating that the htaccess file could be missing. Once I uploaded that file from my own computer (I had saved off all the blog files before upgrading) permalinks and comments worked again!