Something weird is going on with the Google search index. I am seeing widespread reports of SEOs and webmasters complaining that their URLs and web pages are being removed from the Google Index. Lots of folks are noticing that a nice percentage of their web sites are no longer coming up for a site command or showing in Search Console as being indexed.
You can use the fetch feature and once you do that, the page should return back in the index almost immediately but if 50% of your pages have been dropped, that might take a while to do.
A WebmasterWorld thread has a ton of complaints and I am seeing tons of threads in the official Google Search Console forums as well. And yes, folks on Twitter are noticing issues and posting as well.
This is not just about pages not showing up for site commands, it seems like pages are not ranking anymore and traffic is being impacted.
I suspect this is some sort of bug with Google's index but we have not heard anything official from Google.
For now, just know, you are not alone and I will try to dig up more information for you all.