You'll need to run myisamchk to make sure your tables aren't corrupt. The actual error is this bit:

GT::SQL::error = Failed to execute query: '
SELECT *
FROM gforum_Category, gforum_Forum
WHERE gforum_Forum.cat_id_fk = gforum_Category.cat_id
ORDER BY cat_sort_rank ASC, forum_sort_rank ASC
' Reason: Got error -1 from table handler

I can't remember what -1 is off the top of my head.

http://www.mysql.com/...general_options.html