Font Awesome version 5 changed some icon names, and introduces new prefixes fab, far, and fas.

UI/UX Design by

We make sure that your target audience has the best possible experience with your digital product. You get:

• Design tailored to your audience
• Proven processes customized to your needs
• An expert team of experienced designers

Read more Show archive.org snapshot

There is a JavaScript shim that you can use as a "quick fix". It allows you to keep v4 icon names on v5.

The linked page includes details on using that, and a migration guide including a list of icon renames.