There are many reasons to lose traffic. Only a SEO audit can help you find the causes.
I invite you to check your meta robots, robots.txt, xml sitemaps to make sure everything is correct on this side.
Then go to Google Search Console to see if you have been hit by a manual action from Google.
Check also all the reports provided by GSC. Fix all the listed errors if relevant.
Our free ebooks can help you too: https://www.seopress.org/support/ebooks/
And this blog post: https://www.seopress.org/newsroom/featured-stories/what-to-do-when-your-site-disappears-from-google/