If you have received such a notice, read it carefully. It is probably from the court, issued at the request of the State's Attorney. You are probably being summonsed to court to appear as a witness. If you don’t appear, the case may be lost and you can get in trouble for failing to appear. If you have a truly serious scheduling conflict, call the office listed on the document and tell them about your conflict. If you are unable to resolve this and have an attorney, you should ask your attorney to get involved.
Don’t forget that when you do go to court, you should take with you anything that would be relevant as to prove what happened. For instance, any photographs you have of the scene of the crash or the vehicles would be helpful. Any notes that you took right after should be reviewed with the prosecutor as well.