Confluence Application Navigator is missing, app navigator missing from Confluence

Symptom

The application navigator (top-left 'hotdog' drop-down menu) is missing link to Confluence, causing users to be unable to navigate to Confluence. If you go to https:[url]/wiki, Confluence loads, but there is no app navigator to get to other applications.

In Confluence Admin, System Add-ons, the Atlassian Navigation Links Plugin is disabled, and there is no option to enable it.

Workaround

Contact Atlassian Support, who can restart both JIRA and Confluence, which reloads that plugin.