Category Missing

WordPress 2.6.2 was intially installed on this domain: http://webevangels.com/

Several post were published on this blog since 2 weeks. Now I can’t see categories under Manage> Categories

Category list also missing in Post Editing Pages, but when I click on Most used link all the category appear.

When I add a New Category, it is Visible under Manage > Categories. But when I refresh Manage > Categories it disappear.

I tried to edit existing post by adding category, but no result. All post now turned to uncatgorized.

Also old category link such as webevangels.com/category/technology/ and all tag link such as webevangels.com/tag/funny-pictures now return 404 page.

Before everything was working fine.

I tried changing the theme and deactivating all plugin. But no result.

Can anyone figure what would be the problem?