It looks like your robots.txt file was returning a 5xx error since about the 22nd of February. This kind of error will block us from crawling anything from your site and over time can result in us dropping URLs from your site from our search results. In general, you'd see information about this in the Webmaster Tools crawl-errors section. 

That said, it looks like this has been resolved very recently (today?), and we're able to crawl again. If that's the case, things should return back to normal over time, as we recrawl and reindex your pages. 

Hope it helps!

John