how do i go about getting my ssi benefits reinstated after they were stopped ?

If you can appeal within 10 days of the written decision, you can request a waiver of the implementation, and have your benefits continue during the appeal process. You should appeal the written decision within 60 days.  If it has been more than 60 days, an appeal with a justification for why there was a delay may be allowed.  Appearing in person at your local office would be the best thing in my opinion.  To find your local office, use this link https://secure.ssa.gov/ICON/main.jsp 

You file with Social Security. You must have the support of your treating doctor.