Already resubmitted the front page to Google using Fetch as GoogleBot and Submit to index from Webmaster Tools. Also pinged the page to all main Google local SE. On the other hand, sitemap (or site in general) cannot be in sandbox, as it appears in more than 5000 daily searches :). And I have more than 1000 pages in index. And they all appear when I search for the main keyword of each and every page :). The only issue is the main page.
Now, before it disappeared, I have noticed another strange behaviour. My front page displays, randomly, one article from a specific table in my database. Every time I was adding a new article, again the front page was gone from searches for a day or 2. Then everything was back to normal.
Still, my question is this: If something is wrong, why Bing and Yahoo are displaying my front page for the main keyword? And Google not? Somehow, the algorithms are not that different.