If your site has been penalized or banned by Google search engine how do you know that. I am trying to give you some point so that you can easily find out whether your site penalize or banned.
Site is penalize: If your site have been panalize by google search engine in that case your site will not visible for any keyword, keyword phrase or title in search engine result page but when you enter your domain name such as example.com in google search box, your site will be appear, but the question is why google penalize any site, the main cause is you optimized your site in unethical way or follow black hat seo methods such as hidden links, one pixel image, keyword stuffing or having bad link neighbourhood or irrelevant links.google search engine wants to improve his search and try to show best results for user relates quires. If your site get panalize so ask yourself why google think that your site is not relevant to search quiries.
Site is banned: When you types your site on google search box and you are unable to see any results that means google gets excluded your site from its index that is known as banned. Again the question is Why would Google ban a site? Mostly, because that site violates Google Webmaster Guidelines or site webmaster uses blackhat SEO methods for optimizing the site. If your site has been banned by google no problem you can solve this problem,firs remove all black hat seo stuff and resubmit your site to google for indexing.
I request you my friends we care full about above facts before optimize your site. If you follow which hat seo methods and try to take back link in natural way, your site wiil get good reputation amoung the user as well as good rank and stay long.