My Lenovo tablet was on boot loop, no way to shut it down or to access any menu, reset button was ineffective, the only way to stop it was to disconnect the power cord. When I reconnected it the boot loop started all by itself, there was no battery charging logo, I tried ALL tricks describes on the subject on the internet.I was on my way to go to the shop and buy another tablet!
I happen to have another Lenovo tablet, I took off the loaded battery (a difficult operation as it was glued on the tablet ) and put it in the faulty tablet and everything started as normal, without factory reset or any loss of data!. So if the battery is totally exhausted it cannot boot nor load. Strangely enough I put the exhausted battery on the other tablet and did not try to switch it on, it started to load normally, and now both tablet work fine! I hope this can help someone!