Varnish Cache is an open-source web application accelerator (also referred to as an HTTP accelerator or caching HTTP reverse proxy). Varnish stores (or caches) files or fragments of files in memory; this enables Varnish to reduce the response time and network bandwidth consumption on future, equivalent requests. Unlike web servers like Apache and Nginx, Varnish was designed for use exclusively with the HTTP protocol.

In some cases when Varnish Cache is activated, due to a Magento 2 and Varnish issue, the navigation menu can disappear. 

In order to solve this Magento issue, you can remove the TTL attribute (remove ttl=”3600″) from /vendor/magento/module-theme/view/frontend/layout/default.xml, and the navigation menu will appear.