Worried about my daughter

My six year old daughter keeps asking me every now and then what will happen when I (mommy) die and who will takecare of her. She always has a sad thinking face whenever she asks me that. Today she was asking what happens when I go somewhere and never return and I kept telling her I will always return for her but she kept insisting me asking what if what if you don’t return. I don’t know why she is asking me all this and what and how to answer such questions:(