Have you added a Google sitemap lately? If so I would guess this is the problem. I ahve a site added a sitemap, all pages went away, along with my ranking. Took map off pages back and indexed in 3 days results back on page one. To make sure this was not a fluke, I added the site map back, lost all pages. Took it off, guess what? all pages back and indexed well. sitemap section showed no errors, belt or pins. I am on page one again, test over. Dump that baby as fast as you can. If need be you can hand submit your site through CEO.