Oh my goodness my heart breaks for you.. Hannah is a beautiful little girl.
Right now you will be in absolute shock still, do you have answers as to what caused your daughters sudden death? (I'm not asking to know, but it might in time help you to process and will help with relieving the feelings of "could I have done something more?").
Take each day one at a time, be kind to yourself, drink plenty of water or tea or juice, any fluid you can stomach.. if you can, try and eat but it's normal to completely lose your appetite.
Accept any and all offers of help, especially if people want to take your older daughter out. If you are struggling to sleep, speak to the doctor who will prescribe something to help.
Be kind to yourself, do whatever you need to do to get through each day, hour, minute.
Grief is the most painful, overwhelming and personal experience to try and navigate.
There's nothing I can say to take away the horrific pain that you will be feeling but please know that you are in my thoughts and so is your beautiful Hannah.