First of all, track down the factors that got your site banned/penalized; and remove them asap. Secondly, contact Google SPAM/website reconsideration team by signing in to Google Webmaster Tools; and request them to reconsider your site Good luck,
Do all the above but realise how unlikely it is that Google will change its decision. Get over it and move on to you next site.
Google Penalized when One things happened : Two or more website has same onpage content( duplicate website) + Address is same also