Directory categories not displaying

We've had the directory plugin running for some time with no problem but recently the categories stopped showing.
We're using the '[dr_list_categories style="grid | list"]' shortcode but all I'm seeing when I check the code where it should be is an empty list. I'm also not seeing a 'Categories' link under 'Listings' in the admin menu (I haven't played with this plugin much so I'm just sort of assuming it's supposed to be there).
Has anybody else seen a similar problem or have any suggestions for a fix?

thanks