An antique cedar chest—particularly if it belonged to a family member—is a time capsule from the past. It's wise to call a locksmith to open a family heirloom to avoid doing damage. However, if you can't afford a locksmith, or if you bought the chest at a flea market and don't have the key, you can try opening the lock yourself. Be aware, however, that it may cause damage to the lock, the chest, or possibly, the contents.
Go to an antique store, suggests Lockpicking 101. Some antique shops have boxes full of old keys, and one of them might fit the cedar chest. The antique shop might also have an old cedar chest roughly equivalent to yours, and you could inspect its key to see what kind would likely fit.
Buy a blank key from a locksmith, says Fred Taylor of the Worthpoint website. Black it with candle soot or a black magic marker. Insert the blank into the lock, wiggle it around, and pull it out. This will put a mark on the blank where the notch should be. A key to fit the lock can be cut at these points with a hacksaw.
Write the manufacturer, if you can identify one. Ask if you can order a replacement key. Some cedar chest manufacturers can make one for you if you can identify which model you have.
Fall back on force, if finesse has failed you. However, never use a coat hanger wire if the chest is a family heirloom. Lane Furniture suggests that if the chest has exterior lid hinges, unscrew them and open the chest from the back. That way, you'll do no damage to the most visible area of the chest, and if something is wrong with the lock, it can be mended from the inside.