FK can you check (after the sad event) exactly Where she bit off her arms? If she bit them off very close to the buccal tissue (the mouth) there is a very good chance she has autophagy disease. It's nothing you did wrong if this is the case, (not that I think you did anything wrong anyway) but it is HIGHLY contagious and you will need to start again and sterilise the tank and equipment (contact me if this is the case and I'll tell you what to do).
If she has stumps then is it simply old age (sad but not so nasty) and your tank will be fine.
from what i saw when she last came out she bit off the tips of about 3 of her arms and the rest were really corkscrewed. and she also had a twitch sort of thing. and one of her arms looked liked it was completely dead and was pale.