There should be a spring that has apparently become unhooked. Look along the underside of the length of the rod to which the key is attached. You should be able to find a straight (usually blue steele) spring that is supposed to be hooked around at attachment to the rod. A small crochet hook should work to bring it back to where it's supposed to be. If you can't find the spring, it may be missing, if so, you'll need to have it repaired.