I had a similar experience with a Blood Shrimp.
It seems as though mine went through a "bad molt." I was told by a LFS owner that I really trust for advice, when they change their shells, or molt, they don't take precautions like some other inverts (finding a cave, hiding in rock crevice, etc.).
Unfortunately, if there is something aggresive around (in my case a really mean CBS), then they can either hurry the molt and lose body parts, or just have them nipped off by the predator while soft from the molt (occasionly happens with blue-leg hermits).
Try and make sure that you get food to the Blood Shrimp, as it may not be able to grab anything floating by without all of it's legs and claws. If possible, you might want to put it in a separate tank (maybe a QT), while it tries to recover and molt again.
HTH