Manual actions are rarer than the industry's folklore suggests, and most traffic collapses attributed to "a penalty" are algorithmic: a core update reassessing quality, or a technical change that de-indexed a template. The distinction matters because the remedies are opposite — a manual action needs cleanup and a reconsideration request, an algorithmic drop needs the site to become genuinely better.
The first diagnostic step is always the same: open the Manual Actions report in Search Console. If it is empty, stop looking for a penalty and start looking at what changed — a deployment, a robots.txt edit, a migration, a competitor's improvement, or an update date that lines up with the drop.