'Phone them up to let them know you're ill. You can have two sick periods of no more than fourteen days without a sick note each twelve months. If it's longer than two weeks, you'll need a sick note (statement of fitness for work is the proper name) and you can't carry on claiming JSA if it's longer than thirteen weeks.
If you don't make contact within the five working days following the signing date, your claim will be closed down regardless of the reasons for not attending.
All you need to do is complete a form called a JSA28 and in effect, you're self-certifying your incapacity.