Simple and easy fix
Our test model has not been used for 2 years and no longer turns on.
What we did was take its charger and plug it into a USB 3 port on a computer. USB 3 ports are BLUE, so if you have a computer with USB 3 ports they will look just like normal usb ports only BLUE. leave it for several hours then try to turn it on.
Alternatively you could use a high powered USB charger. You may find leaving it on charge for 12+ hours in the normal charger resolves the issue, either way its likely your main battery has died so be prepared for the battery in the tablet to not hold a charge for very long.