It seems like there was a "minor" bug impacting some of the reports for the Sitemaps indexing count.
A Google Webmaster Help thread had a couple of webmasters come in asking why Google was reporting that all of a sudden, the index count was down to zero.
As you can imagine, having an index count of zero can be very stressful even if you are still getting traffic from Google.
Many people rely on Google Webmaster Tools for the reporting and to determine the health of a site in Google's index.
Google confirmed their was a "minor bug" that has been fixed to address the sitemaps reporting error. Googler, Asaph said:
We have looked into the issue your reported with sitemaps for arizonarealtyexpert.blogspot.com and uncovered a minor bug in our reporting system. You should see accurate numbers in a few days. Thank you for bringing this to our attention. Note that this is just a reporting issue, it does not impact actual crawl or index of your site.
Forum discussion at Google Webmaster Help.