Acting once you know the cause.
IF IT WAS TECHNICAL
Fix it, then request indexing for important pages. Recovery is usually quick once the obstruction is removed.
IF IT WAS A MIGRATION
Check every old URL redirects to its new equivalent, in one hop. Missing redirects lose the ranking those URLs held.
Export old URLs from analytics or Search Console and check each.
IF A NOINDEX OR ROBOTS BLOCK WAS LEFT IN PLACE
Remove it, resubmit the sitemap, and request indexing for key pages. Recovery typically takes weeks.
IF IT WAS AN ALGORITHM UPDATE
There is no switch to flip. The response is to improve the site: better content, better usability, removing thin pages.
Recovery, if it comes, takes months and follows genuine improvement rather than tactics.
IF IT WAS A MANUAL ACTION
Search Console states the reason. Fix the underlying issue and submit a reconsideration request explaining what you did.
IF IT WAS COMPETITION
Uncomfortable but honest: someone produced something better. The response is to be better, or to target queries where you can win.
