Does anyone have any idea why my guinea pig may died?

One of my 3 guinea pigs mysteriously died with no explanation what so ever the night before she was perfectly fine as normal, there was nothing obviously wrong, when i opened the door to feed her she escaped (the hutch is at very low to the ground so she didn't fall) which wasn't unusual as they often push against the door when im trying to feed them. I simply picked her up and put her back in the hutch and she was fine. However next morning i went to see them and she had ped away in the night and i had no idea what had happened, she had no injuries to suggest the others had attacked her and they had lived together for two years with no problem. she was about 2 1/2 years old when she died.