I got 2 of these used, and one wouldn’t power up at all. I opened them both, disconnected both batteries and cross-connected the good battery to the one that wouldn’t boot, The cable is short but if you hold them back sides facing each other it can be done. Sort of like using booster cables to start your car :)

That enabled me to boot the tablet, and it went into windows setup mode (the other tablet was Android). then I connected the bad battery to the good tablet, charged it there for an hour and then connected everything back. After that both tablets came back up as expected.

Some say that just disconnecting the battery can help. It didn’t in my case.

Be careful, there are 2 screws near the connector for the keyboard, remove those too before using your nail or a guitar pick to pry it open. There are videos on youtube showing how to do that