Thanks for the replies. I have never tried flashing the bios, so I dont think that can just brick / not work (but yes, anything can happen 🙂 )
Regarding the battery – the days in standby might have broken the battery, but it is still possible to charge it and turn the tablet on (it will get very hot) and off (PWR in 10 sec and it will become cool again). Nothing is ever shown on the screen.
Anyway I will have to contact Banggood to get an replacement