The Hidden Triggers Behind Your Website’s Traffic Plunge—and How to Beat Them Fast
5. See if you received a manual action
Google can take manual actions to demote your website, usually for breaking their spam policies. Manual actions may remove your website from the search index entirely.
Check Security & Manual Actions → Manual Actions in Google Search Console to see if your site has any manual actions.

This report will tell you if and why a Google employee has penalized your content directly.
Most often, manual penalties are related to violating Google’s guidelines, especially regarding spammy content. If that’s the case, reviewing those guidelines can help you fix the issue: