You get a message in WMT saying your site has been penalized for unnatural backlinking.
You need to download from WMT all your links and sift out the rubbish, depending on how many you have you can get rid of PR n/a and OBL >15 for a start. Look for links on completely unrelated sites and check out the diversity of your archor text and fix if necessary. Use google disavow tool to list all the backlinks you want removed.
It takes a good 4 weeks plus to see results but way better than starting a new site generally.
BTW, the keywords your targeting are they difficult or real easy ?