Quick Launch missing from SharePoint subsite after deleting links

The Quick Launch navigation has disappeared from one SharePoint subsite (Marketing) after I deleted some placeholder links on a Marketing page.

It was working previously, and still appears on other subsites in the same site collection. In Site Settings, Quick Launch is still enabled, but there is no option to add or edit navigation links anymore, and the left navigation is completely gone on this subsite.

Has anyone seen this before or know how to restore the Quick Launch for just one subsite?

Thank you!