There are tons of concerns and complaints on Twitter and on Google Webmaster Help that the index count in the Google Search Console sitemap report has drastically dropped. Meaning, Google is reporting that the pages Google has indexed of your web site has dropped.
Here is my report:
You can see a drop of about 8,000 pages or 25% drop in the pages Google has indexed of my site.
I suspect this is an error, a bug, and that it really doesn't mean Google dropped 25% of my site from their index. Why? Because everyone else is noticing a similar issue.
And the threads I linked to above.
Google has not commented on the issue yet but I would not worry.
Update: John Mueller from Google says it is likely a bug:
@Marie_Haynes Yes, as far as I can tell it's just a reporting bug. Sorry for the confusion! We're checking with the folks to get things back— John Mueller (@JohnMu) May 13, 2016