Most professionals who repair music boxes will tell you that repairing a music box is a difficult task. If it is an heirloom or an expensive antique, do not attempt to repair it unless you have considerable mechanical skills and are familiar with how the mechanisms work. However, most professionals will recommend the simple repair of releasing the tension from the main spring to undo over-winding.
Take off the lid that covers the working parts. Usually this is a glass or plastic inset. Be careful to collect your screws for later.
Push the start button to the right. There will be a connected metal plate inside of the box that also moves to the right. This will cause the starter mechanism to move to the right as well, thus releasing the latch from a small hole embedded in the biggest cylinder wheel.
Watch for the air brake to start turning--it will whirl around, catching the light. This indicates that your coil is unwinding. After the coil is finished unwinding, turn it a bit to see if your machine will work. If it still doesn't work, consult a professional.
Replace the cover to protect the machine from dust and debris.