Do you guys have any idea why the main site for eg, say xyz.com would completely drop out of Google Webmaster Tools charts like this? I have never seen anything like this. It just looks like the site doesn't exist anymore on these charts but I know we're still getting hits. This is starting to make us a bit nervous.
Check if the your index links are still the same in numbers or not. Probably your index links are removed by Goolge.
- If not make sure your page rankings and backlinks are not ceased.
If you have recently added low quality external links to your sites
remove it and crawl the pages again.
I had this scenario not so long ago, when without me knowing, redirect from HTTPS to HTTP URLs that we had for long accidentally got removed and 2/3 of our URLs got indexed with HTTPS instead of HTTP. And we had a HTTP version of the site registered with Google Search Console. So, naturally, data was dropping on our property on GSC until I figured what the problem was and registered a HTTPS version of the site with Google Search Console, and started seeing our traffic on there...