sudden death of four year old Silver Martin

Question
This won't bring her back, but recently an adopted rabbit died in our home.  We got her from a friend who was moving and couldn;t take her along.  She had only been with us for three months, though had been fine with our friend for three years or so. She seemed well the morning of and then when we returned homw after being out for two hours, found her dead in her cage.  She was on her side and had her legs stretched out in front and behind.  There was an odd bony protrusion as if it were her scapula on her left side ( the side she was lying on) and a soft squishy protrusion coming from that side as well.  Her diet had not changed.  Everything seemed to be normal.  I need to contact our friend to tell her of the loss, but know that she'll wonder why just as we do.  Thanks for any light you can shed.  

Answer
hi merfoard i'mafraid its just one of those things that can happen -one moment they look ok and the next they have passed away-rabbits live on average about 3-6 years so in rabbit terms it was a old rabbit when you consider that rabbits are classed as adults at 5 months old.
dave.