First ,try contacting the institution or agency where you studied. Even if they don't have your records on file , they can refer you to someone who does.
Many times the programs are sponsored by a local high school. Your local education department will have this information.
Contact your local State Department of Education. Most telephone directories list government listings in a special section. There should be listings there for education.
If all else fails the United States Department of Education can most certainly help you out.
There is a record of your achievements out there. It may take a little effort, but you can get a copy of your diploma.
If they only offer a transcript of your achievements, you may be able to use the transcript to have a certificate or diploma created .