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 overwinding.
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.
If you are dealing with a music box that has been in storage for a long time, take it to a repair shop to check if it is capable of running and that the original lubricants in the machine have not hardened.
Pay careful attention that you do not damage or touch parts in your music box that cannot be replaced without considerable cost such as the combs. Combs for antique music boxes have not been manufactured for more than 100 years and will have to be retooled by hand and can cost a couple of hundred dollars per tooth. If you are working on an antique music box, do not touch the comb with your bare hands or it will cause it to oxidise, which will eventually cause the wheels or disks to stick and damage them considerably. Never remove any part from your music box without releasing tension in the main spring. A wound tension spring will cause parts to fly off the machine and cause severe damage.