Fatal error: Call to undefined function get_category_id() in /home/vertical/public_html/wp/wp-content/themes/VideoTheme/VideoThemeRes/index.php

Solution:

As the error points out, get_category_id() isn’t a valid WordPress function.

You’ll need to use get_cat_ID() instead.

The only way this could have worked before is if that custom function was defined in functions.php or a plugin that was updated.