Hello. My name is Marc. I am a licensed attorney, and I will be happy to answer your question.
In order to be summoned to court, a person has to receive proper Notice. I write Notice with a capital "N" because it has a very important meaning in legal proceedings. If a person does not receive legal papers, including a letter from the court, with proper Notice, then the person should not incur any consequences for failing to respond.
In your case, the court might have sent it to your last known address. If you can provide proof that you no longer reside there, then there should be no consequences for failing to appear on the scheduled date.
So, what should you do now? If the letter contains a phone number, call it and explain the situation. These things happen often, and whatever it is that you missed can be rescheduled. Just make sure to give them your proper address. If there's no phone number, try to find out what Part in the court the letter was sent from. Or see if the letter has the name of a judge or other court personnel. Then, look up the number for the court and ask for the appropriate person.
Again - even lawyers sometimes miss court appearances. It's usually not a problem as long as you take action now and communicate with the court. I trust this will resolve well for you.
I hope I have provided you with useful information and helped you to better understand your issues and options. Please let me know if you have any further questions. Otherwise, please feel free to rate my answer, as that is the only way I can receive credit.
Good luck and best wishes,