My guinea pig died what was the cause?

so I just found my guine pig dead in her cage, she was looking fine this morning and last night. Her left side of her belly was sunken in and either her eyes were to or the other guinea pig ate them. The only thing I can think of that was a little odd this morning is that her hair was sticking up a bit more than normal (she is an albino american guinea pig) please help me because I would like to know why she died! (she lives with another american guinea pig, they are both female)