Push the coin return button. This will frequently cause a stuck coin to fall out of the chute and come back out through the coin return.
Insert tweezers into the coin slot and maneuver them to see if you can grasp the coin that is lodged inside and pull it out.
Knock on the area around the coin slot with your hand. The vibrations will sometimes be enough to loosen the stuck coin.
Don't strike the machine too hard, though, to avoid damaging it or your hand.
Drop another coin into the slot. Sometimes a second coin falling will dislodge the first one.
Acquire the key for the coin box from the owner of the machine or proprietor of the establishment where you are doing laundry. Opening the coin box can cause the stuck coin to become dislodged.
Call a service technician if the coin is still not coming out. This should only be done if you are the owner of the machine, since service calls typically cost several hundred dollars.