PHP error: call to member function on non-object?

I just started getting the following error:
Fatal error: Call to a member function on a non-object in /../../../wordpress/wp-blog-header.php on line 234
Line 234 is:
$dogs = $wpdb->get_results("SELECT * FROM $tablecategories WHERE 1=1");
I haven’t changed that file at all. I had just changed some include() paths in other files, but I can’t see how that would affect this. I looked at the file, and I can’t see any errors. I have tried reuploading the file several times, but it isn’t making any difference. The administration interface works fine, but when I go to the index page I get this error. The site is radioactiverabbit.com.