It Could be that Google has erased the cache of your web.

This was opened with my new domain. I resubmit the sitemap and ping the URL.. within 4-6 days it was back.. Also see if you accidiently not apply for the URL removal request..

I thinks that it's completely depends on search engine's crawling and indexing process.

that means your website is not cached by google, check the webmaster tool for further instruction or information.

If its a new domain or page, then it might take some time to index it. You can bookmark your site to speed up the process.