I'm not sure if this can help, but you can try doing a force reset by pressing the power and volume up button at the same time until there is a logo pop up on the phone. If that doesn't work, then I believe the motherboard might be broken. I found an ebay link that has a working 16 GB motherboard: https://www.ebay.com/itm/334225280740
A repair at a shop may cost around $150-300.