To be honest with you, once your site got flagged or slapped by Google, even if you managed to recover, the fact that your site got slapped won't get erased from your domain's history. In other words, it is much easier, faster and more economical to start over rather than fixing your already penalized site.
Move your content to a new domain and start your SEO campaigns from beginning. Be careful of blackhat techniques. Always try to get high quality links from relevant sites.
Pay attention to social media marketing as well.
All the Best!