My daughter found her "Sammy" hamster dead yesterday. She was bloody, don't know what could've happened.?

Other than not sleeping in her little house as usual, she was fine. She had bled, maybe from her or ? We had her for 3 years but still can't explain all the blood. She was well taken care of; fed, clean and l-o-v-e-d! Nothing in her cage could've hurt her. My daughter is 12 and very devastated.