Windup toys work by turning a crank, creating tension inside the toy's inner components. As the tension is released, gears and sprockets turn, causing movement in the toy. Because the inside of every windup toy has a different design, the only way to repair the toy is to open it up and identify which parts are broken. This is a simple process, although it requires you to be comfortable working with small components.
Open the toy. Inspect the toy to determine how to open it. Almost all wind up toys consist of a front and back half that are held together by a series of tabs and slots. Using a flat screwdriver to bend the tabs up will allow you to open the toy. Newer toys made out of plastic may have slots without tabs, in which case you simply press the screwdriver into the slot to release it.
Carefully analyze the inside of the toy. In most cases, the component that is damaged or broken will be clearly visible. Windup toys use simple mechanical motion consisting of a spring and gears. Look over these parts to determine which is broken.
Remove the part and take it with you to find a replacement. Small springs and gears can typically be found at hardware stores, electronics stores or hobby supply shops. Make sure you get a replacement part that is identical to the original so that it will fit properly. If you cannot find the proper part, you may need to seek out vintage toy repair shops and suppliers to find an original part from a similar broken toy.
Replace the component inside the toy. The parts should pop into place without the need of specialized tools. Give the toy a small test wind while it is still open, so that you can see the movement of the parts and make sure that everything is turning properly.
Reassemble the toy. Place the two halves back together and bend the tabs down. If you are working with a newer toy that doesn't have tabs, the two halves of the toy should click, or snap, back together easily.