Two causes of a fall in search traffic differ from the rest. A security issue means Google found your site was hacked or could harm visitors, for example through malware or phishing. A spam problem means the site breaks Google's spam policies, its rules against deceiving people or manipulating search results.
Search Console has a report for each. A manual action is applied after a person at Google decides pages break the spam policies. This guide shows how to read both reports and what to do if they show something.