Yellow Bellied Slider Turtle died........?

I have 3 yellow bellied slider turtles for over 7 years. I fed them yesterday and when I went to check on them this afternoon. One of them was dead. Is it common for this breed of turtle to die unexpectedly like this. They are well cared for and the only thing I can think of is that she choked or was ill and I didn't know. Does anyone have any information so as to put my mind at rest as I am feeling very guilty and sad. Not knowing what happened to her is really upsetting me.