My Facebook Messenger is not showing, how do I fix it?

With Shopify's addition of the Facebook Messenger, a minor code change will be needed to enable this within your store and make it visible.

For all themes go to your Shopify admin and click on: Themes > Customize Theme Actions > Edit Code

On the Following page click on the Assets folder and choose Stylesheet.css.liquid
If you are using a Mac Hold CMD + F
If you are using a PC Hold CTRL + F

This will bring up a search field where you will enter:
.fb_iframe_widget

This will automatically take you to this piece of code.

/* To fill the container and nothing else */
.fb_iframe_widget, .fb_iframe_widget span, .fb_iframe_widget span iframe[style] {
  width: 100% !important;
}

Just above that, you will also see:

#fb-root { display: none; }

Delete both of these and click save. Once that has been removed this will allow the Messenger chat button to appear on your website.