Death of a bird

My daughter has been keeping a parakeet for three years.
We love her very much. When she is freed from the cage, she flies in my house and she sometimes stops on our shoulders or the top of our head. She is very lovely.

Yesterday she died.
When I came back to my home, I found that she was laying on the bottom of the cage and didn't move at all.
I didn't understand the reason for her death. She sang and flew in the morning on that day. I took care of her in the same way as usual.
My daughter had been attending a ski camp and was absent from home on that day. It is my responsability to take care of her during her absence. I thought I had to apologize to her.