Cartridge won’t stop spilling ink
Try drying the cartridge by putting it on top of some dry tissue and let it spill until it’s dry and won’t spill out ink anymore (this sometimes takes very long time and many tissues).
Cartridge won’t let out the ink
Put the cartridge’s head in hot water or ammonia (or both) for about 2 minutes, clean it up and try again. Make sure not to let water touch the yellow plate in the front of cartridge.
For full explanations : here or here (mirror).