As you would know reptiles shed their skin as they grow, with the old skin being replaced with a new, larger skin. Sometimes reptiles, in particular lizards, will not completely shed their old skin for whatever reason (usually humidity issues or damaged scales) so the old skin remains. As the animal grows the old skin will become tighter and tighter until eventually the blood circulation is cut off and the limbs simply die. Think of it as if you were still wearing baby socks, they would be so tight that no blood would be able to get to your feet. That is probably what has happended here. Check your remaining geckos to ensure they aren't having any problems shedding, particularly around the feet/toes and tail. Everytime an animal sheds you should check to make sure all of the old skin has come off, if it hasn't then you need to remove it yourself by soaking the animal in tepid water until the skin is loose enough to be peeled off gently. It should only be done when the skin is loose, ripping it off if it is still stuck tight will cause more damage then good.