the March update was a core update, which means that usually it's a content related issue. Not always, but usually...
Therefore, audit your content and either improve, or rewrite / restructure it in some way to add more value to it...
Also, check which pages lost traffic or rankings, and either point some internal links to those pages if they didn't have links pointed to them yet (point links from other related pages but which are ranking / getting organic traffic), or outright delete those pages altogether if their content is really bad / duplicate / not targeted at any topic or keyword. Sometimes, deleting pages that don't perform well can salvage your site, but deleting the pages should be a last resort. Focus on improving / rewriting / restructuring / updating the (penalized) content first, or doing some internal linking and if these things don't bring your site back within days (ok, give it 1 week, just to be sure) then you can delete the non-performing pages.
And if you do delete those pages, don't forget to also 301-redirect their URLs to other relevant pages, or to the homepage / category pages that make most sense, so you don't end up with 404 pages which are bad for SEO