Over the past few days, there were several reports in the Google Webmaster Help forum that the structure data report within Google Webmaster Tools was reporting zero elements of structured data.
Of course, for those who spent the time and resources adding rich structured data to their site, this can be concerning.
It appears it was simply a reporting bug and had no impact on the structured data showing up in the search results as rich snippets. Nevertheless, the issue seems to be resolved and the structure data reports have returned back to normal for most.
Yesterday, Google's John Mueller update webmasters in the thread saying:
Thanks for your patience, everyone! It looks like this should be resolved in the meantime.
The reporting of the structured data found on your site wouldn't be related to any change you'd see in your site's visibility in search, so if you're seeing changes there, that would generally be due to something else (and you might want to start a thread separately about it).
Forum discussion at Google Webmaster Help.