The court has wide discretion in imposing contempt charges. The punishment can range from a verbal warning to jail. You can only "plead the 5th" if answering the question might cause to admit guilt of a crime. You can't "plead the 5th" to avoid taking the stand in court in front of a defendant who terrifies you. If this is a criminal matter, I'd call the prosector that issued the subpoena and go in and talk to him/her about your concerns. Also, check in with the victims rights co-ordinator at your court to see if they can help with your situation.
Bottom line: if you don't show, a bench warrant could be issued for your arrest, the sherrif's deputies will come find you and bring you in front of the judge. At that time a hearing on contempt will take place and the judge can verbal admonish you, fine you or jail you. I'd be proactive and seek out the prosecutor that subpoena'd you and explain your situation.