Disclaimer, read first. I have no qualifications to give any advice on this topic. I give no assurances or guarantees of any kind. This article is not meant to serve as an instruction, it's just a description of what I did. Whatever you do, do it at your own risk, after properly studying and following safety measures.
I have a Gigabyte GZ-G90B1 Power Bank which seems to be more commonly known as RF-G90B. It's also appears to be the same as Enerpad MG-9000 which is the brand name for Samyatech contents (board + cells imported from Japan, or so they say). So Gigabyte just sells that.
I haven't used it for a while and when I tried to use it, it appeared dead. No matter if I tried to charge it, use it to charge mobile, press the button and/or hold it, the device wouldn't react and lights would stay off.
As I've already lost my receipt I had no hope for replacement, so I unscrewed it (there's a single screw). The box wouldn't open at first because the cells are glued to both halves of it, but there's no way around it but to pull. I detached the chip from the cells and then attached it back. This seems to have rebooted the chip and it started working.