At this point you can either file a motion to reopen the I-751 denial, or if your case gets referred to Immigration Court, then you can renew your I-751 before the immigration judge. You definitely need to consult with an experienced immigration lawyer at your earliest convienence.
Best of luck.
Verdin Law Firm, LLC
Your situation calls for a professional immgration attorney to fully reevaluate your filing as certain things may have triggered an issued denial from the USCIS.
It appears that something might have triggered a closer scrutiny of your particular case and to effectively continue your case it needs attention of an immigration attorney.