If you don't show up for jury duty a bench warrant will be put on you. Basically when a bench warrant is issued, the law enforcement officers have the authority to pick you up and bring you to court to address the charges of a bench warrant. Visit http://www.essortment.com/law-definitions-bench-warrant-53966.html for more information.
It depends on where you live. Most places will issue some kind of warrant and you will have headaches and fines and then they will still make you serve. Some cities will also suspend your renewal on your vehicle registration until you serve or are excused.
Check with your local jurisdiction for what they consider automatic excusals - you may have a legit excuse and not know it.
You will get in trouble for breaking the law. Imagine if no one ever showed up when they were supposed to for anything? What if it was your day in court for something important to you but nobody showed-up?