I think many of your valuable links has lost or got deleted by sites. Losing backlinks may hurt your visibility and traffic big times, especially if your site doesn’t have loads of backlinks overall. If the traffic drop affected most keywords and pages, check for any noticeable changes to your backlink profile from any tool like Moz, Ahrefs, SEMRush, SEO SpyGlass or etc. If there's a big drop seen in Backlink Progress graph then you can check your lost backlinks by checking Last Found/Seen Date in next columns which will give a hint as to when you may have lost any.
If you're hit by any Algorithm Update or Manual Search Engine Penalty then you must check it in your Webmaster's and fix the onsite or offsite related issues and then request Google to review the site.