PHP function not defined causing menus not to show below 768px

Hi,

Unfortunately I keep getting disconnected from chat. Several people (Nastia, Patrick and Luis) have started to look into this for me but once disconnected I can't get them back :slight_frown:

I know they were able to access the chat logs from my query, so hopefully you will be able to as well.

A brief summary:

I have inherited this WP site, but am not a WP person at all.
I have toggled for responsive design in the theme options.
When I do this the following error is displayed:
Fatal error: Uncaught Error: Call to undefined function dropdown_menu() in /homepages/29/d553433818/htdocs/wp-content/themes/modernize/header.php:172

I know enough coding to at the very least comment this line of code out, and, when i do, a strange thing happens. The error is resolved, the website loads but, when scaling below 768px, the nav menu disappears.

Steps we've taken so far:
- backed up WP, updated all plugins and theme
- edited config to turn on debugging log
- given WPMUDEV access and shared FTP details

This is as far as we've gotten. Any help is extremely gratefully received!