【答案解析】Dear Anna,
Thank you for your invitation. Unfortunately, it is a regret that I can"t go with you because of a sudden illness falling upon me a few days ago.
I caught a serious cough with a continuous fever last week and the doctor asked me to stay in bed for a week. Therefore, I can"t attend the appointment. I feel really sorry for not meeting you at the theater and the inconvenience it will bring to you. I hope we can arrange something else afterwards. If so, please don"t hesitate to send me a message about your preferable date.
I hope you can understand my situation and accept my apology.