Mobile menu disappeared - been told by customer service that it is because of CSS we have

The menu on our mobile site has suddenly disappeared, I've spoken to customer service and they said it is because of CSS we have on the site. 

The CSS has always been there as was input by the web developer that built the site.

I just changed the style of the main menu because there wasn't room for it to work with the logo in the middle, this appears to be what triggered the issue with the mobile menu disappearing.

Hope someone can help!

 

@Retrofit-Assist Would it be possible to share your site's URL so I can take a look? Thanks!

Yeah, it's https://retrofitassist.ca

You have this code in your Custom CSS or code injection, if you remove it then the mobile menu will reappear:

.header-burger-btn {
    display: none;
}