wordpress widgets screen disappeared

This is both a change in WordPress core in 3.5, and a problem specific to your commercial Theme.

In WordPress 3.5, if a Theme does not register support for dynamic sidebars (i.e. Widgets), then the WP-Admin does not display the Appearance -> Widgets admin page. So, if the behavior changed between WordPress 3.4 and 3.5, or if the screen re-appears if you switch to Twenty Twelve/Eleven/Ten, then the problem is that your chosen Theme doesn’t support Widgets.

Specific to your commercial Theme, iThemes builder, this is a known bug, and one that was apparently fixed. If you need any additional assistance with this commercial Theme, you should contact the developer, iThemes.