If Bing Webmaster Tools reports a significant increase in 'Pages Not Indexed', what is the most critical factor to investigate first to determine the root cause?

The most critical factor to investigate first is whether the robots.txt file is inadvertently blocking Bingbot from crawling these pages. The robots.txt file is a text file on your web server that tells search engine crawlers which pages or sections of your site they should not access. A sudden and significant increa....