Go to google webmaster tools and go to this link https://www.google.com/webmasters/tools/reconsideration choose your site and be honest and write what happens with you after 1 or 2 week google may reindex you again
i think some one of your competitors report your site as spam if you are num#1 so you should make reconsideration to get your site back on Google search
Google Webmaster tools help you to get the current status of your website....and help to correct it if there is something wrong...try it.
yeah, just check the webmaster tool, if you were removed from the index, there will surely be a notification on your google webmaster tool.
Add your site to Google Webmaster Tools if you have not done that already and use the reconsideration option after confirming that everything on your site is fine and there is nothing like virus or spam.