The worst case scenario is hold you in contempt of court. This is highly unlikely but an attorney cannot advise someone not to abide by a subpoena if properly served. I would contact the Attorney who issued the subpoena and give them the reason why you cannot attend the trial.
It is not unusual for a District Attorney to have the court issue a warrant for your arrest for failure to appear. The easiest way to handle the problem is to contact the prosecutor and tell him/her that you can be there. Most times, the prosecutor will attempt to assist a witness. If not, you have to be there or face the punishment.