sites go into sandbox for suspicious activities or when google is not sure where or how to rank them. idea is to exercise ethical link building practice on a site with unique comments and take is easy with link building. if any site goes into sandbox, then don't panic or give up but keep building backlinks steadily and just wait. time is the best judge here
re-index your site. submit it again to google, build quality links and don't do black hat. Google will notice that and take you out of the sandbox.