The battery might be the issue.
I had a scenario a year or so ago when my Insignia NS-P08W7100 Windows tablet would shut down at 80% battery charge. Eventually the tablet would not start without the charger being connected. Later, it would not reliably start even with the charger connected.
The battery simply wore out. even though the battery was fully charged, it could not suply enough power to keep the device from shutting down. I ended up tossing it into a recycling bin.