Why did my rabbit die?

i had a 10 year old netherland dwarf rabbit, she died last night and i just dont know why. i was upstairs watching tv when i heard a squeaking, it sounded like a dogs chew toy so i thought my daog was messing around down stairs, but when i went down stairs i found my rabbit in her cage with her moulth open squeaking and twitching i tryed to comfort her by rubbing her ears and the top of her head and then she quieted down and passed. can anyone tell me what happened to my poor bunny?