Anthony
Welcome to the forum, you might want to have a look at the following FAQ which explains where everything is
Welcome to Benefits and Work I'm afraid it is likely that your file will now be returned to the DWP for failing to attend and you will need to explain to them why you did not.
Have you had problems with postal deliveries before? Is there a postmark on the envelope that shows when it was posted? What was the date on the letter itself?
Gordon