The report is in Google Webmaster Tools within Sitemaps, the place Google suggests webmasters look to check accurate index saturation count.
One user said:
Since yesterday all my web indexed pages turned to 0.
My sitemap is reachable and readable? and marked green in webmaster tools.
I made no changes in webmaster tools.
I wouldn't panic, there are lots of similar reports now. Personally, all the sites I checked were fine. So either Google just fixed it or it is not affected all Sitemap files.
Again, this should just be a reporting glitch and it should be fine.
I should note that Google has not confirmed this as a bug yet, so I cannot be certain.
Update: Google confirmed this is a bug and is only a reporting glitch.