Manual action vs algorithmic hit
Start in Search Console under Manual Actions. If there is a message there, you have a manual action: a reviewer penalised your site, usually for unnatural links, thin or spammy content, or cloaking. If that report is clean but traffic still fell, you have an algorithmic adjustment, no message, just a re-weighting by an update.
This distinction decides everything. Manual actions need a fix plus a reconsideration request. Algorithmic hits cannot be appealed; they recover only when you genuinely fix the underlying quality and Google re-evaluates.