forums have disappeared and add book page not visible

Assuming that you're using the core Forum module.

As I experienced while upgrading to 5.x, the new Category module istalls the Taxonomy and Book wrappers automatically while enabled first time, i.e. replacing these modules with it's own interfaces for Category system. To use old Taxonomy data, which the Forums are based on, you need to uninstall Taxonomy-wrapper from the Category settings page. But to be honest, it doesn't make any sense to run Category and Taxonomy simultaneously, as both serve the same purpose.

The same with Book - If you want the original functions again, uninstall Book wrapper. But since you said you already imported the books, you should use Category to create more pages.

So, the thing you need is to import your Forums vocabulary as a Container into the Category module, which is done by Category_legacy module (you'll be able to turn this one off again, after the import is done).

!!! IMPORTANT !!! This import normally doesn't work well with Forums! But I recently created a patch for category_legacy module, enabling this import to work fine. See this isuue with my patch: http://drupal.org/node/158683

I'm sorry to mark this as "duplicate", but there's no point posting the patch to more than one location. You can re-open if my patch and explanation doesn't solve it.

if i can see your post before i use category module, my forums will not die.

thanks.