What happens to a witness when they are subpoena to appear in court for testimony?

I would work closely with the prosecutors office. The prosecutors office has a right to subpoena you. If you are subpoenaed as a witness, you must appear in court. The judge wI'll hold you in contempt  if you do not appear. However, if you are in danger I would put that on the record with the prosecutor to protect yourself. You will protect yourself with both the prosecutor and the judge if there is a written record of the danger you are in.