1. Check the input USB if it charges.
2. Check the output USB if it can charge another device.
3. Check the power bank button if any.
4. Connect the input USB to the output USB.
5. Observes if it indicates charging.
6. Get phone repair tools.
7. Loose the power bank as shown here.
8. Desolder the battery wires from the panel.
9. Carefully separates all the battery.
10. Check the battery specifications and test each of them. Eg 3.7V/1000mAh.
11. Replace any battery that is totally dead.
12. Plug the panel to charger and trace fault if is from panel.
13. Most of the problem power bank has is either from.
a) Battery total dead.
b) Input USB fault needs replacement.
c) The voltage directional diodes fault.